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>

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