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

include/starkernel/console.h

@@ -129,14 +129,38 @@ void console_puts(const char *s);
 void console_println(const char *s);
 
 /**
- * console_ensure_line_start - Make sure the next console_putc() starts at
- * the beginning of a fresh output line, emitting a newline if output is
- * currently mid-line (e.g. dangling behind a re-anchored prompt).
- * No-op if already at line start. Mirrors console_putc()'s dual
- * serial+framebuffer behavior (a bare '\n' reaches both).
+ * console_ensure_line_start - Make sure the *next real output* starts at
+ * the beginning of a fresh output line, closing off whatever is currently
+ * mid-line (e.g. a dangling prompt). No-op if already at line start.
+ *
+ * FABRIC-2.md §I.9 fix, 2026-09-05: the newline is DEFERRED, not emitted
+ * immediately -- it only actually reaches the console on the next real
+ * console_putc() call, and is silently dropped (never emitted at all) if
+ * console_cancel_deferred_line_start() is called first instead. Before this
+ * fix, an immediate, unconditional newline here meant any caller invoking
+ * this function "just in case" (sk_repl_idle() being the one real caller)
+ * would visibly snap a bare, unfinished prompt line to a fresh blank line
+ * even when nothing was actually about to be printed -- indistinguishable
+ * from Enter having already been pressed at that prompt. Deferring means a
+ * caller that turns out to have nothing to print can cancel cleanly, with
+ * zero visible effect, while a caller that does print gets the correct
+ * "close the old line first" behavior for free, still counted by
+ * console_tx_count() as real output (unlike the old always-silent inner
+ * form) since it is realized through the normal console_putc() path.
  */
 void console_ensure_line_start(void);
 
+/**
+ * console_cancel_deferred_line_start - Discard a pending deferred newline
+ * from console_ensure_line_start() without ever emitting it. No-op if no
+ * newline is currently deferred. Callers that speculatively deferred a line
+ * break before checking whether they actually have anything to print
+ * (sk_repl_idle()'s idle-beat check being the motivating case, FABRIC-2.md
+ * §I.9) call this when the check comes back negative, so the console is
+ * left exactly as it was -- no stray newline, no phantom blank line.
+ */
+void console_cancel_deferred_line_start(void);
+
 /**
  * console_tx_count - Monotonic count of console_putc() calls delivered to
  * either output (serial and/or framebuffer). In use by the REPL to detect