Fix MergeItemSequence shutdown deadlock

[BCook98]

Summary

• Transfer MergeItemSequence continuation ownership while holding the state mutex, then resume only after unlocking.
• Keep the terminal state sticky across cancellation and upstream completion races.
• Cancel and await the detached cross-process update-log reader after a successful native pool close.

The 1.16.1 implementation can deadlock when close cancels the update-log reader while its poll task finishes and resumes the same waiting task under the mutex. This was reproduced as a hung absolute-path PowerSync database close.

Tests

• 1,000 cancellation-versus-upstream-finish races.
• 200 concurrent absolute-path open and close cycles.
• Full package suite: 112 tests passed.