[codex] Add durable MEMFS snapshotting for SQLite sync

[ashfame]

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Branch note

This branch has been rebuilt directly on top of trunk. It intentionally does not include the opfs_sqlite_flush_risk branch stack or the old VACUUM INTO snapshot path.

What changed

• Added durable byte-level filesystem snapshot, diff, and restore primitives for MEMFS.
• Added snapshot publishing helpers for OPFS and S3-compatible object storage.
• Integrated MEMFS snapshots into OPFS SQLite flushing so .ht.sqlite plus journal/WAL sidecars can be persisted from live bytes.
• Exposed snapshot and restore methods through PHPWorker for client-side remote publishing flows.

Why

This creates a correctness-oriented snapshot substrate for SQLite sync, including journal and WAL mode sidecar files, and gives callers a direct path to publish snapshots to S3-compatible storage without relying on OPFS as an intermediate.

Validation

• npm exec nx test php-wasm-universal -- --runInBand
• npm exec nx test playground-storage -- --runInBand
• npm exec nx test php-wasm-web -- --runInBand
• npm exec nx test playground-remote -- --runInBand
• npm exec nx lint php-wasm-universal
• npm exec nx lint playground-storage
• npm exec nx lint php-wasm-web
• npm exec nx lint playground-remote
• npm exec nx run php-wasm-universal:typecheck
• npm exec nx run php-wasm-web:typecheck
• npm exec nx run playground-remote:typecheck
• npm exec -- tsc -p packages/playground/storage/tsconfig.lib.json --noEmit