fix(sync): flush pending CRDT write-backs on shutdown instead of dropping them

[h4yfans]

Summary

On shutdown, CrdtProvider.destroy() called cancelPendingWritebacks(), which cleared the debounced (500 ms) markdown write-back timers without running them. Any note/journal edit made within ~500 ms of quit never reached the on-disk .md file or the search index. The CRDT state is persisted (so the edit isn't permanently lost), but the file silently lagged the true content and did not self-heal — only a new edit re-triggered a write-back. For a "files on disk, not a database" product, that on-disk/UI divergence (which also propagates stale via folder sync and stale search results) is a real correctness gap.

This flushes pending write-backs on shutdown instead of cancelling them.

Closes #543.

Changes

• crdt-writeback.ts — pendingTimers now stores { timer, doc } per pending note (was just the timer handle, which a flush couldn't reach). New exported async flushPendingWritebacks(): snapshots pending entries, clears their timers, and runs every performWriteback to completion; per-note failures are caught and logged, so the flush never rejects. cancelPendingWritebacks() is retained (still used elsewhere).
• crdt-provider.ts — destroy() now await flushPendingWritebacks() as its first line, before the entry.doc.destroy() loop (the flush reads from the live Y.Docs). destroy() is already awaited from stopSyncRuntime() → the Electron before-quit handler, so the async flush is awaited on the real shutdown path.

Tests

Added 4 cases to crdt-writeback.test.ts (fake timers, so the only way a write happens is the flush):

1. flush runs a scheduled write-back without advancing the debounce timer
2. flush clears pending timers so they do not fire a second time
3. flush with nothing pending is a no-op
4. a failing write-back during flush is logged, not thrown (flush still resolves)

Two existing test files that mock ./crdt-writeback and exercise destroy() were updated to add flushPendingWritebacks to their module mocks (crdt-provider.test.ts also flips its destroy() assertion to the new function).

Verification

• pnpm --filter @memry/desktop typecheck:node — exit 0
• pnpm exec vitest run --config config/vitest.config.ts --project main for the 3 sync test files — 44 passed (incl. the 4 new)
• pnpm lint — exit 0 (0 errors)

Notes

• Docs gate skipped (MEMRY_DOCS_IMPACT_SKIP=1): internal shutdown-sequencing correctness fix; no public API or user-facing config, and no existing doc describes the prior drop-on-quit behavior.
• Deferred (filed separately if found): the same drop-on-quit shape may exist for other debounced projections; this PR covers only the markdown write-back.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
memry-docs	Building	Preview, Comment	Jun 12, 2026 9:59pm
memrynote-landing	Building	Preview, Comment	Jun 12, 2026 9:59pm

[github-actions]

No React Doctor issues found in this scan.

Score	Issues	Errors	Warnings	Affected Files	Scope
100 / 100 (Great)	0	0	0	0	6 files changed on fix/flush-writebacks-on-shutdown vs. main

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