FABRIC.md item 4.4x: split the REPL prompt into its own bottom strip

Scope expanded from pure CANVAS-rectangle arithmetic (as originally
scoped) to also splitting the REPL prompt/input line out of the
scrollback box into an independent single-line strip, per Captain Bob's
explicit fold-in after the gap was reported (§25.0 rule 3) rather than
silently expanded.

vt100.c: VT100_BOX_ORIGIN_X/Y are no longer hardcoded per-arch literals --
both are now derived from fb_width()/fb_height() at vt100_enable_ttf()
time. New vt100_strip_draw() renders the bottom strip (gray border lines,
bright-white text) directly via the existing ttf_draw_glyph_cell()
rasterizer, independent of the box's own grid/cursor state. Border lines
are drawn after the glyph loop so an oversized cell can only be clipped
by them, never erase them.

console.c/console.h: console_fb_strip_draw() thin wrapper, matching the
existing console_fb_enable_ttf()/console_fb_scroll_*() pattern.

repl.c: builds a plain-text "[VMName] ok> <input>" mirror in
g_strip_prompt/strip_refresh(), refreshed on every keystroke (including
backspace) from sk_readline() -- already wired for item 4.4v, since
keyboard-typed characters will flow through the same console_getc() path
once that lands. Also widened sk_repl_step()/sk_repl_run()'s local input
buffer from a second, smaller 256-byte buffer to INPUT_BUFFER_SIZE
(1025), per 4.4w's decision.

Verified: three-arch clean QEMU boot + logs, amd64 screendump showing
the box and strip as two visually distinct regions with no visible
glyph/border clipping.

Punch list §25 item 4.4x complete.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/starkernel/hal/console.c

@@ -317,6 +317,13 @@ void console_fb_scroll_fwd(uint32_t n)
     }
 }
 
+void console_fb_strip_draw(const char *text)
+{
+    if (fb_is_available()) {
+        vt100_strip_draw(text);
+    }
+}
+
 /**
  * Read a single character from serial console (non-blocking)
  * Returns -1 if no character available

src/starkernel/hal/vt100.c

@@ -45,6 +45,7 @@
 #include "starkernel/kmalloc.h"
 #include "log.h"
 #include <stdint.h>
+#include <string.h>
 
 /* Maximum CSI parameters */
 #define VT100_MAX_PARAMS 16
@@ -82,23 +83,40 @@ static ttf_raster_cache_t  g_ttf_cache;
 
 /* -----------------------------------------------------------------------
  * FABRIC.md item 4.4t: confine REPL text (TTF mode only) to the 640x480
- * CANVAS box computed in 4.4o and pixel-verified in 4.4p. Per-arch raster
- * top-left origin, from those items' measurements. Boot/POST (bitmap mode)
- * is untouched -- g_origin_x/g_origin_y default to (0,0) and only change
- * once vt100_enable_ttf() runs.
+ * CANVAS box computed in 4.4o and pixel-verified in 4.4p. Boot/POST
+ * (bitmap mode) is untouched -- g_origin_x/g_origin_y default to (0,0)
+ * and only change once vt100_enable_ttf() runs.
+ *
+ * FABRIC.md item 4.4x: origin is now DERIVED from fb_width()/fb_height()
+ * at vt100_enable_ttf() time rather than hardcoded per-arch literals --
+ * both the old per-arch numbers (320/104 amd64, 80/4 aarch64+riscv64) and
+ * the new ones below check out against the same formula, so this is a
+ * strict generalization, not a behavior change for X. Y changes because
+ * 4.4u/4.4w replaced 4.4m's 96px/4-line REPL strip with a single-line
+ * strip pinned to the bottom of the screen (see VT100_STRIP_* below) --
+ * the box now sits directly above that strip instead of assuming 96px.
+ *   origin_x = (fb_width()  - VT100_BOX_W) / 2          -- horizontal center
+ *   origin_y = fb_height() - VT100_BOX_H                -- box bottom edge
+ *              - VT100_STRIP_BOX_GAP_PX                 -- gap above strip
+ *              - (2*VT100_STRIP_GAP_PX + 1)              -- strip height
  * --------------------------------------------------------------------- */
-#if defined(__x86_64__) || defined(__i386__)
-#define VT100_BOX_ORIGIN_X 320u
-#define VT100_BOX_ORIGIN_Y 104u
-#else
-#define VT100_BOX_ORIGIN_X 80u
-#define VT100_BOX_ORIGIN_Y 4u
-#endif
 #define VT100_BOX_W    640u
 #define VT100_BOX_H    480u
 #define VT100_BOX_COLS 53u  /* 640 / VT100_TTF_CELL_W_PX, exact */
 #define VT100_BOX_ROWS 20u  /* 480 / VT100_TTF_CELL_H_PX, exact */
 
+/* -----------------------------------------------------------------------
+ * FABRIC.md items 4.4u/4.4w/4.4x: the REPL prompt/input strip -- a
+ * single-line region pinned to the bottom of the screen, independent of
+ * the scrollback box's own grid (see vt100_strip_draw() below). Numbers
+ * pinned by Captain Bob directly, 2026-08-12 (4.4w).
+ * --------------------------------------------------------------------- */
+#define VT100_STRIP_GAP_PX      15u  /* baseline-to-border-line gap, both sides, mirrored */
+#define VT100_STRIP_BOX_GAP_PX   8u  /* gap between the strip's top border and the box's bottom edge */
+#define VT100_STRIP_BORDER_GRAY  FB_RGB(0xAA, 0xAA, 0xAA)  /* FB_ANSI_PALETTE[7] */
+#define VT100_STRIP_TEXT_WHITE   FB_RGB(0xFF, 0xFF, 0xFF)
+#define VT100_STRIP_LEFT_MARGIN_PX(fbw) (((fbw) - VT100_BOX_W) / 2u)  /* aligned under the box */
+
 static uint32_t g_origin_x = 0;  /* box top-left, TTF mode only; 0 in bitmap mode */
 static uint32_t g_origin_y = 0;
 
@@ -268,6 +286,68 @@ static void ttf_draw_glyph_cell(uint32_t cell_px, uint32_t cell_py, uint8_t ch,
     }
 }
 
+/* FABRIC.md items 4.4u/4.4w/4.4x: draw the REPL prompt/input strip -- a
+ * single-line region pinned to the bottom of the screen, independent of
+ * the scrollback box's own grid and cursor state (this function never
+ * touches g_vt, and nothing in the box's own put_char()/scroll_up() path
+ * touches the strip). Reuses ttf_draw_glyph_cell() for the actual glyph
+ * blit -- same rasterizer, same cache, as the box's own grid -- rather
+ * than a second text-drawing path.
+ *
+ * Horizontal scroll (4.4u step 4): @p text is drawn tail-anchored once it
+ * exceeds the strip's width. That is equivalent to cursor-anchored
+ * scrolling because the caller (repl.c) only ever appends at, or
+ * backspaces from, the end of the line -- there is no mid-line cursor
+ * movement in this REPL, so "keep the cursor visible" and "keep the tail
+ * visible" are the same window.
+ *
+ * Border lines are drawn LAST, after the glyph loop, not first: a glyph
+ * cell's own opaque background fill (cell_h()=24px) can extend a few
+ * pixels past the strip's 15px baseline-to-line gap for full-ascender
+ * characters -- 4.4u's own record already flags these gap numbers as
+ * "informal... need to be re-checked once actually built." Drawing the
+ * borders last means an oversized cell can never erase them; it can only
+ * be visually clipped by them, which is the correct failure direction.
+ *
+ * No-op before TTF mode is active. */
+void vt100_strip_draw(const char *text)
+{
+    uint32_t fbw, fbh, bottom_line_y, top_line_y, baseline_y, cell_py;
+    uint32_t margin_x, avail_px, avail_cols, len, start, i;
+
+    if (!g_ttf_ready || g_glyph_mode != VT_GLYPH_TTF) return;
+    if (!text) text = "";
+
+    fbw = fb_width();
+    fbh = fb_height();
+    bottom_line_y = fbh - 1u;
+    top_line_y    = bottom_line_y - (2u * VT100_STRIP_GAP_PX);
+    baseline_y    = bottom_line_y - VT100_STRIP_GAP_PX;
+    cell_py       = baseline_y - VT100_TTF_SIZE_PX;
+    margin_x      = VT100_STRIP_LEFT_MARGIN_PX(fbw);
+
+    /* Clear the whole strip first -- both border lines get redrawn every
+     * call regardless, so clearing border-to-border is simpler than
+     * clearing just the interior and no less correct. */
+    fb_fill_rect(0, top_line_y, fbw, bottom_line_y - top_line_y + 1u, VT100_DEFAULT_BG);
+
+    avail_px   = (fbw > 2u * margin_x) ? (fbw - 2u * margin_x) : 0u;
+    avail_cols = cell_w() ? (avail_px / cell_w()) : 0u;
+
+    len   = (uint32_t)strlen(text);
+    start = (len > avail_cols) ? (len - avail_cols) : 0u;
+
+    for (i = start; i < len; i++) {
+        uint32_t col = i - start;
+        ttf_draw_glyph_cell(margin_x + col * cell_w(), cell_py,
+                             (uint8_t)text[i],
+                             VT100_STRIP_TEXT_WHITE, VT100_DEFAULT_BG);
+    }
+
+    fb_fill_rect(0, top_line_y,    fbw, 1u, VT100_STRIP_BORDER_GRAY);
+    fb_fill_rect(0, bottom_line_y, fbw, 1u, VT100_STRIP_BORDER_GRAY);
+}
+
 static void draw_cursor_glyph(uint8_t ch)
 {
     uint32_t f = eff_fg();
@@ -391,8 +471,12 @@ void vt100_enable_ttf(void)
      * scrollback allocation block just below depends on g_vt.cols via
      * g_line_stride, so origin/cols/rows MUST be set first or every
      * shadow/ring index silently corrupts instead of crashing. */
-    g_origin_x = VT100_BOX_ORIGIN_X;
-    g_origin_y = VT100_BOX_ORIGIN_Y;
+    /* FABRIC.md item 4.4x: derived from the real framebuffer size, not a
+     * hardcoded per-arch literal -- see the formula in this file's header
+     * comment above VT100_BOX_W. */
+    g_origin_x = VT100_STRIP_LEFT_MARGIN_PX(fb_width());
+    g_origin_y = fb_height() - VT100_BOX_H - VT100_STRIP_BOX_GAP_PX
+                 - (2u * VT100_STRIP_GAP_PX + 1u);
     g_vt.cols  = VT100_BOX_COLS;
     g_vt.rows  = VT100_BOX_ROWS;
     g_vt.cx = g_vt.cy = 0;

src/starkernel/repl.c

@@ -35,6 +35,7 @@
 #include "starkernel/timer.h"
 #include "starkernel/arch.h"
 #include <stdint.h>
+#include <string.h>
 
 /* FABRIC.md 4.4: "ok>" (including its trailing space) renders in bright
  * cyan, 0x55FFFF -- reuses FB_ANSI_PALETTE[14]. Sent as a real SGR escape
@@ -44,6 +45,72 @@
 
 const char lithos_version[64] = LITHOS_VERSION_STR;
 
+/*===========================================================================
+ * FABRIC.md items 4.4u/4.4w/4.4x: the bottom REPL strip -- a plain-text
+ * (no ANSI/SGR) mirror of "[VMName] ok> <input so far>", refreshed into
+ * console_fb_strip_draw() on every keystroke. Built separately from
+ * SK_PROMPT_TEXT rather than derived from it, since the strip renders in
+ * one fixed color (4.4u) and has no SGR parser of its own -- the ANSI
+ * version keeps going to console_puts() exactly as before, unaffected,
+ * for serial and for the scrollback box's own history.
+ *===========================================================================*/
+
+#define SK_STRIP_PROMPT_MAX 80u
+
+static char g_strip_prompt[SK_STRIP_PROMPT_MAX] = "ok> ";
+
+/* Builds "[VMName] ok> " or "[VMName] zuse)ok> " (no VM name -> just the
+ * suffix) into g_strip_prompt, bounded, no strcat -- mirrors emit_prefix()'s
+ * bracket format (console.c) so the strip and the box agree on what a
+ * prompt looks like, minus the ANSI color codes the strip doesn't use. */
+static void set_strip_prompt(VM *vm)
+{
+    const char *vn = console_get_vm_name();
+    char *p = g_strip_prompt;
+    size_t room = sizeof(g_strip_prompt) - 1u;
+    size_t n;
+    const char *suffix;
+    size_t used, left, sn;
+
+    if (vn && vn[0]) {
+        n = strlen(vn);
+        if (n > room) n = room;
+        *p++ = '[';
+        memcpy(p, vn, n);
+        p += n;
+        *p++ = ']';
+        *p++ = ' ';
+    }
+
+    suffix = (vm && vm->zuse_session) ? "zuse)ok> " : "ok> ";
+    used = (size_t)(p - g_strip_prompt);
+    left = (used < room) ? (room - used) : 0u;
+    sn = strlen(suffix);
+    if (sn > left) sn = left;
+    memcpy(p, suffix, sn);
+    p += sn;
+    *p = '\0';
+}
+
+/* Concatenates g_strip_prompt + input_so_far (bounded, no strcat) and
+ * hands the result to the strip renderer. Called once with an empty
+ * input_so_far right after set_strip_prompt(), then again on every
+ * keystroke from sk_readline() below. */
+static void strip_refresh(const char *input_so_far)
+{
+    char combined[INPUT_BUFFER_SIZE + SK_STRIP_PROMPT_MAX];
+    size_t plen = strlen(g_strip_prompt);
+    size_t ilen = strlen(input_so_far);
+
+    if (plen >= sizeof(combined)) plen = sizeof(combined) - 1u;
+    memcpy(combined, g_strip_prompt, plen);
+    if (plen + ilen >= sizeof(combined)) ilen = sizeof(combined) - 1u - plen;
+    memcpy(combined + plen, input_so_far, ilen);
+    combined[plen + ilen] = '\0';
+
+    console_fb_strip_draw(combined);
+}
+
 /*===========================================================================
  * USE-word dispatch: which VM receives REPL input.
  *
@@ -90,6 +157,9 @@ static int sk_readline(char *buf, int size)
 {
     int n = 0;
 
+    buf[0] = '\0';
+    strip_refresh(buf);   /* FABRIC.md 4.4x: show the bare prompt before any input */
+
     for (;;) {
         int c = console_getc();   /* non-blocking poll */
 
@@ -128,10 +198,12 @@ static int sk_readline(char *buf, int size)
         /* backspace: DEL (0x7F) or BS (0x08) */
         if ((c == 0x7F || c == '\b') && n > 0) {
             n--;
+            buf[n] = '\0';
             /* VT100 erase: move back, overwrite with space, move back again */
             console_putc('\b');
             console_putc(' ');
             console_putc('\b');
+            strip_refresh(buf);
             continue;
         }
 
@@ -139,7 +211,9 @@ static int sk_readline(char *buf, int size)
         if (n >= size - 1) continue;  /* buffer full — drop character */
 
         buf[n++] = (char)c;
+        buf[n] = '\0';
         console_putc((char)c);        /* echo */
+        strip_refresh(buf);
     }
 
     buf[n] = '\0';
@@ -180,7 +254,7 @@ static void sk_fault_handler(VM *vm) {
 
 int sk_repl_step(VM *vm)
 {
-    char input[256];
+    char input[INPUT_BUFFER_SIZE];   /* FABRIC.md 4.4w: matches the strip's input width */
 
     if (!vm || vm->halted) return 0;
 
@@ -194,6 +268,7 @@ int sk_repl_step(VM *vm)
         int is_hera = (!vn || (vn[0]=='H' && vn[1]=='e' && vn[2]=='r' && vn[3]=='a' && vn[4]=='\0'));
         vm->emergency_console = is_hera ? (vm->zuse_session ? 0 : 1) : 0;
         console_puts(SK_PROMPT_TEXT);
+        set_strip_prompt(vm);   /* FABRIC.md 4.4x: bottom strip mirrors the same prompt */
     }
 
     sk_readline(input, sizeof(input));
@@ -222,7 +297,7 @@ int sk_repl_step(VM *vm)
 
 void sk_repl_run(VM *vm)
 {
-    char input[256];
+    char input[INPUT_BUFFER_SIZE];   /* FABRIC.md 4.4w: matches the strip's input width */
     VM  *active;
 
     vm->halted = 0;
@@ -241,6 +316,7 @@ void sk_repl_run(VM *vm)
             int is_hera = (!vn || (vn[0]=='H' && vn[1]=='e' && vn[2]=='r' && vn[3]=='a' && vn[4]=='\0'));
             active->emergency_console = is_hera ? (active->zuse_session ? 0 : 1) : 0;
             console_puts(SK_PROMPT_TEXT);
+            set_strip_prompt(active);   /* FABRIC.md 4.4x: bottom strip mirrors the same prompt */
         }
 
         sk_readline(input, sizeof(input));