starkernel: REPL scrollback, ~1000 lines (FABRIC.md item 4.4q)

Scope decided with Captain Bob before implementation: ring buffer +
recall on today's full-screen vt100 grid, not also confining REPL text
to the 4.4o 640x480 box (that confinement stays open as its own future
item, not a third silent deferral). No keyboard input path exists yet
(M8 unstarted), so the trigger is two new FORTH words, SCROLL-BACK
( n -- ) / SCROLL-FWD ( n -- ), exercised via serial injection.

vt100.c gains a text-only 1000-line ring buffer (kmalloc'd, tens of KB
-- not pixel snapshots, which would be ~1000x larger for no benefit)
plus a shadow buffer mirroring the current screen. scroll_up() now
pushes evicted rows into the ring before the pixel scroll. History is
one continuous sequence (ring then shadow); scrolling always redraws
from that sequence -- no separate pixel-scroll path for scrollback,
decided up front to avoid retrofitting later.

New src/word_source/scroll_words.c (Module 31), thin wrappers over
console_fb_scroll_back()/_fwd() -> vt100_scroll_back()/_fwd(). Bug
caught during live testing: both words initially used an off-by-one
underflow check (dsp < 1) copied from a different, older dsp
convention elsewhere in this codebase; vm_pop() (which these words
actually call) uses dsp as a 0-based top-of-stack index, so the check
rejected every legitimate single-argument call. Fixed by removing the
separate precheck and relying on vm_pop()'s own guard.

Live-verified on all three architectures (exceeds this item's
amd64-minimum bar): generated 50+ lines via a FORTH loop, confirmed
SCROLL-BACK recovers correctly older content, and on amd64 confirmed
SCROLL-FWD returns to genuinely live state (not a frozen snapshot) by
showing the injected commands' own echo. Known limitation confirmed by
direct pixel measurement: redrawn lines lose their original SGR color
(not stored per-cell) -- text recovers exactly, color does not.

Three-arch verified: Failed: 0, dict-hashes identical across all
three (values changed correctly from prior items -- two new words
were added).

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

include/starkernel/console.h

@@ -82,6 +82,14 @@ void console_fb_init(const FramebufferInfo *info, FbPixelFormat fmt);
  */
 void console_fb_enable_ttf(void);
 
+/**
+ * FABRIC.md item 4.4q: thin wrappers over vt100_scroll_back()/
+ * vt100_scroll_fwd() -- see those functions' doc comments for the full
+ * contract. No-op if the framebuffer console was never initialized.
+ */
+void console_fb_scroll_back(uint32_t n);
+void console_fb_scroll_fwd(uint32_t n);
+
 /**
  * Write a single character to serial console
  */

include/starkernel/vt100.h

@@ -67,12 +67,29 @@ void vt100_init(void);
  * Lazily loads the font capsule and its raster cache on first call
  * (no-op on later calls). Recomputes cols/rows for the new cell size and
  * clears the screen, since the two glyph backends use different cell
- * dimensions. Provisional TTF point size/cell dimensions -- FABRIC.md
- * item 4.4m owns the final decision. No-op if the font capsule fails to
- * load (stays on font_8x16.c; logged, not fatal).
+ * dimensions. TTF point size/cell dimensions decided final by FABRIC.md
+ * item 4.4m (20px text, 96px REPL strip). No-op if the font capsule
+ * fails to load (stays on font_8x16.c; logged, not fatal).
  */
 void vt100_enable_ttf(void);
 
+/**
+ * FABRIC.md item 4.4q: move the REPL scrollback view back/forward by
+ * @p n lines and redraw. Offset 0 (the default, and where every call
+ * eventually returns to) is the live view -- the same content already on
+ * screen. Clamped at both ends: back cannot pass the oldest stored line,
+ * forward cannot pass the live view. No-op (including no redraw) if the
+ * scrollback buffers failed to allocate at vt100_enable_ttf() time, or
+ * before TTF mode is active at all.
+ *
+ * Known limitation: redraw uses the terminal's current default fg/bg,
+ * not each line's original SGR color state at the time it was printed
+ * (colors aren't stored per-cell) -- text content is recovered exactly,
+ * color is not.
+ */
+void vt100_scroll_back(uint32_t n);
+void vt100_scroll_fwd(uint32_t n);
+
 /* -----------------------------------------------------------------------
  * Character output
  * --------------------------------------------------------------------- */