Fix framebuffer console: idle heartbeat corrupts in-progress input line

sk_repl_idle() (called every ~1s from sk_console_readline()'s idle loop)
opens with console_ensure_line_start(), which unconditionally forces a
newline whenever the console isn't at a line boundary -- including
mid-edit, after characters have been typed and echoed but before Enter.
This fired on every elapsed SK_IDLE_BEAT_INTERVAL regardless of whether
sk_repl_idle() had anything to print, visually snapping the in-progress
input line to a fresh empty line -- indistinguishable from Enter having
been pressed. Most noticeable on the space key since it's the most common
key hit during a pause.

Gate the idle beat on n == 0 (no in-progress edit), mirroring the n > 0
guard the prompt reanchor logic just below already uses. Deferring the
xhci/block-sync idle service by at most one more interval while a line
is being edited is within its own documented "coarse cadence, cheap
early-exit" tolerance.

Verified: reproduced via QMP send-key against a live amd64 QEMU boot
(multi-character line typed with pauses across several idle intervals
stayed intact after the fix, where it previously broke on each interval).
Re-verified boot to ok> on all three architectures (amd64/aarch64/riscv64)
per repo acceptance policy.

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

src/starkernel/repl.c

@@ -548,7 +548,19 @@ int sk_console_readline(char* buf, int size, VM* active_vm, int reanchor_prompt)
             heartbeat_service();
 
             uint64_t now = heartbeat_ticks();
-            if (now - g_last_beat_tick >= SK_IDLE_BEAT_INTERVAL) {
+            /* n == 0 gate: sk_repl_idle() opens with console_ensure_line_start(),
+             * which unconditionally forces a newline whenever the console isn't
+             * already at a line boundary -- including mid-edit, with characters
+             * already typed and echoed (n > 0) but Enter not yet pressed. Without
+             * this gate, that forced break fires on every elapsed
+             * SK_IDLE_BEAT_INTERVAL regardless of whether sk_repl_idle() actually
+             * has anything to print, visually snapping the in-progress input line
+             * to a fresh (empty) line -- indistinguishable from Enter having been
+             * pressed. Deferring the whole idle beat while a line is being edited
+             * (same n > 0 guard the prompt reanchor below already uses) delays
+             * xhci/block-sync servicing by at most one more interval, which its
+             * own "coarse cadence, cheap early-exit" design already tolerates. */
+            if (n == 0 && now - g_last_beat_tick >= SK_IDLE_BEAT_INTERVAL) {
                 g_last_beat_tick = now;
                 sk_repl_idle(active_vm);
             }