FABRIC-2.md §I.9: fix the terminal phantom-linebreak defect

console_ensure_line_start() (hal/console.c) used to emit its newline
immediately via a path that deliberately skipped the tx-byte counter, so
that sk_repl_idle()'s unconditional per-beat call to it (repl.c, ~1s idle
heartbeat) could force a real newline the REPL's own prompt-reanchor logic
never noticed -- the prompt was never reprinted, and the next real
keystroke echoed onto the now-blank line, indistinguishable from Enter
having already been pressed at a bare prompt. Root-caused in the previous
commit (704573b); this commit applies the fix per explicit go-ahead.

Fix: defer the newline instead of emitting it eagerly.
console_ensure_line_start() now only sets a flag (g_pending_line_close);
the newline is realized -- for real, and counted by g_console_tx_count
like any other output -- on the next actual console_putc() call, or
silently discarded via the new console_cancel_deferred_line_start() if the
caller decides nothing was actually printed. sk_repl_idle() captures
tx_before_idle right after its console_ensure_line_start() call and cancels
the deferred newline at both of its exit points when console_tx_count()
hasn't moved. A beat with nothing to report now leaves the console
untouched; a beat that does print still closes the dangling prompt line
first, properly counted this time. The pre-existing n > 0 mid-edit gate in
sk_console_readline() is untouched -- independent purpose, not the bug.

Verified via the mandatory foreground 3-arch QEMU acceptance boot
(clean qemu, amd64 -> aarch64 -> riscv64, one at a time): all three reached
(zuse) ok>, all echoed the first real input on the same log line as the
prompt rather than a fresh line, all shut down cleanly via BYE. amd64's log
additionally shows live human backspace-correction still glued to the same
prompt line. Logs: logs/20260905-015854 (amd64), logs/20260905-020248
(aarch64), logs/20260905-020552 (riscv64); logs/20260905-015813 is a
foreground-rule-violation retry killed and redone correctly, kept per this
project's "never delete logs/" convention.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

src/starkernel/hal/console.c

@@ -260,16 +260,33 @@ static void emit_prefix(void) {
  * Also mirrors output to the framebuffer VT100 terminal when available.
  * Serial output is ALWAYS active regardless of framebuffer state.
  *
- * console_putc_inner(): the same body minus the tx-counter increment.
- * console_ensure_line_start() uses the inner form so its format-only
- * newline does not read as "real output" to the REPL's prompt re-anchor
- * (repl.c) -- a silent idle beat that merely closes a dangling prompt line
- * must not be mistaken for chatter and trigger a prompt repaint. The
- * counter counts characters the caller actually intended to emit.
+ * console_putc_inner(): the same body minus the tx-counter increment --
+ * used internally for the newline console_putc() itself realizes out of
+ * g_pending_line_close below (see console_ensure_line_start()), so that
+ * one real character write emits at most one counted "start" (the '\n'
+ * plus the character together read as a single unit of real output, not
+ * two, to console_tx_count()'s callers).
  */
 static void console_putc_inner(char c);
 
+/* FABRIC-2.md §I.9 fix, 2026-09-05: set by console_ensure_line_start(),
+ * cleared either by console_cancel_deferred_line_start() (no output
+ * followed -- drop the newline silently) or here in console_putc() (real
+ * output followed -- realize it for real, counted, immediately before the
+ * character that triggered it). See console.h's doc comments on both
+ * functions for the full rationale; the short version is that the old
+ * unconditional-and-immediate version of this newline fired every idle
+ * heartbeat regardless of whether anything was actually about to be
+ * printed, visibly snapping a bare REPL prompt to a fresh blank line on
+ * its own. */
+static int g_pending_line_close = 0;
+
 void console_putc(char c) {
+    if (g_pending_line_close) {
+        g_pending_line_close = 0;
+        g_console_tx_count++;
+        console_putc_inner('\n');
+    }
     g_console_tx_count++;
     console_putc_inner(c);
 }
@@ -313,10 +330,15 @@ void console_ensure_line_start(void)
 {
     if (!g_line_start)
     {
-        console_putc_inner('\n');
+        g_pending_line_close = 1;
     }
 }
 
+void console_cancel_deferred_line_start(void)
+{
+    g_pending_line_close = 0;
+}
+
 /**
  * Write a string with newline to serial console
  */