Queue async monitor update writes synchronously

[tnull]

Monitor update persistence relies on KVStore write calls being queued in call order. The incremental async update path deferred that write until executor polling, allowing async scheduling to reorder persistence operations relative to later updates or cleanup.

Add a regression test that builds an incremental update future without polling it and verifies that the write has already been queued.

Co-Authored-By: HAL 9000

This finding was discovered by Project Loupe

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

The fix is correct and consistent with the persist_new_channel pattern at line 1597; res_a is awaited at line 1633 with a valid 'static boxed future. The test correctly forces the incremental path via maximum_pending_updates = update_id + 1 and asserts synchronous queueing. I have no new issues to add beyond my prior review pass.

No issues found.

[tnull]

Updated to line-wrap commit messages.