Keep ESU collection inside synchronized output - #5495

Problem

When an application sends BSU, printable cells and ESU in one pane input batch, the printable cells may still be pending in the active screen_write_ctx collection when ESU is handled.

screen_write_stop_sync() clears MODE_SYNC and redraws lines already recorded in sync_dirty. The still-pending collection is only flushed later by screen_write_stop(), after synchronized mode has ended, so those cells can be written to the physical client outside the outgoing CSI ?2026h / CSI ?2026l transaction. The final pane grid is correct, but a capable terminal may display the frame progressively instead of atomically.

This is distinct from #4983, which covered structural TTY writes while BSU and ESU span multiple pane reads, and from #5419/#5420, which concerned cursor visibility during client redraw.

Fix

Handle application ESU with a screen_write_ctx-aware helper:

1. While MODE_SYNC is still set, flush the active collection. In synchronized mode this records the affected lines in sync_dirty instead of writing them to the client.
2. Then call the existing screen_write_stop_sync() to clear the mode and redraw the complete dirty set inside physical synchronized output.

The existing pane-only stop helper remains unchanged for resize, teardown and redraw callers that do not have an active parser collection.

Test evidence

The regression script is intentionally provided as a PR attachment rather than committed:

• sync-output-atomic.sh
• SHA-256: 53adcc9f8024bfde719724557242d2ebda7109d91d28e4fac15af12c33316da9

It starts an attached sync-capable nested tmux client, sends two complete 80x24 application synchronized-output frames, captures the physical client byte stream, and requires every frame-row marker to occur inside balanced outgoing DEC 2026 transactions.

Results against the same workload:

• master (851c5a933d4838c32ad06c248b2ba975d106149c): fails with marker 000000:00 outside synchronized output.
• This branch: passes repeatedly (5/5).

Additional validation:

• clean ./configure --enable-utf8proc --enable-asan and build
• decrqm-sync.sh
• input-requests.sh
• screen-redraw-cache.sh
• screen-redraw-outside.sh
• tty-draw-line.sh