Persist OPFS SQLite databases via snapshots

[ashfame]

⚠️ DON'T REVIEW YET

Draft PR Description

Summary

This PR fixes OPFS persistence for WordPress sites using SQLite by preventing
transient SQLite sidecar files from being saved alongside the persisted
database.

Previously, OPFS persistence mirrored MEMFS writes at the byte level. That meant
the live SQLite database file and its temporary coordination files could be
copied into OPFS as-is, including files like .ht.sqlite-shm. Persisting those
sidecars can leave a saved site with a stale SQLite state on reload.

The fix changes OPFS persistence so the live SQLite DB and sidecars are excluded
from normal journal replay and initial MEMFS-to-OPFS sync. Instead, Playground
creates a SQLite-cooperative snapshot with VACUUM INTO, validates it, and
publishes only the canonical .ht.sqlite file into OPFS.

Commit Structure

The first commit intentionally adds only the regression test:

1. Add OPFS SQLite sidecar regression test

That commit is expected to fail in CI against the existing implementation. The
test creates a stale .ht.sqlite-shm sidecar before saving a temporary site to
OPFS, then asserts the saved OPFS database directory does not contain that
sidecar. Without the fix, OPFS contains .ht.sqlite-shm and the test fails.

The second commit adds the implementation and supporting unit coverage:

2. Persist OPFS SQLite databases via snapshots

With the implementation in place, the same regression passes because OPFS stores
only the coherent .ht.sqlite snapshot.

Implementation Details

• Adds a platform-level SQLite snapshot routine exposed via
  /internal/shared/snapshot-sqlite.php.
• Uses the SQLite PDO connection to run best-effort
  PRAGMA wal_checkpoint(TRUNCATE), then VACUUM INTO.
• Validates the snapshot with PRAGMA integrity_check.
• Excludes .ht.sqlite, .ht.sqlite-journal, .ht.sqlite-wal, and
  .ht.sqlite-shm from OPFS journal replay and initial MEMFS-to-OPFS sync.
• Adds DirectoryHandleMount.persistSqliteSnapshot() and publishes snapshots
  through a temp file before replacing .ht.sqlite.
• Wires snapshot persistence into OPFS initial sync, explicit flush, unmount,
  and debounced SQLite DB write handling.

Verification

Regression behavior was verified both ways:

• With only the regression test commit applied, the Playwright test fails
  because OPFS contains .ht.sqlite-shm.
• With the fix applied, the same Playwright test passes.

Commands run:

LD_LIBRARY_PATH=/tmp/playwright-deps/usr/lib/x86_64-linux-gnu PLAYWRIGHT_TEST_BASE_URL=http://127.0.0.1:5400/website-server/ npx playwright test --config=packages/playground/website/playwright/playwright.config.ts packages/playground/website/playwright/e2e/opfs-sqlite-flush-risk.spec.ts --project=chromium --retries=0

npm exec nx test playground-remote -- --run src/lib/playground-worker-endpoint.spec.ts --testTimeout=20000
npm exec nx test php-wasm-web -- --run src/lib/directory-handle-mount.spec.ts --testTimeout=20000
npm exec nx run php-wasm-web:typecheck
npm exec nx run playground-remote:typecheck
npm exec nx run playground-website:typecheck
npm exec nx test playground-wordpress
git diff --check

Note: this machine needed Playwright Chromium and local browser shared
libraries. The LD_LIBRARY_PATH prefix points at locally extracted Ubuntu
browser dependency packages used only for this verification environment.