feat(sqlite): add deferred commit mode with flush sequencing

[NathanFlurry]

Stacked on #5659 (sync-sqlite). Opt-in deferred SQLite commit mode: a commit returns after updating actor-local page state, a background flusher ships pages to the engine in order, and callers wait on a monotonic flush sequence. This is the storage-layer primitive for Durable Object style output gates. Design: docs-internal/engine/sqlite/deferred-commits/SPEC.md.

• Add CommitMode::Deferred to the native SQLite VFS: committed pages land in a pinned overlay, a single flusher batches and ships them in order with the existing head fence, and commitSeq/flushedSeq/waitForFlush expose durability with snapshot semantics.
• Keep read-your-own-writes across the overlay, in-flight batch, caches, prefetch, and engine reads.
• Retry indeterminate commits with a byte-identical resend; a fence mismatch, wrong head, retry deadline, worker-close timeout, or flusher failure breaks the database and stops the actor generation so no waiter can observe a lost write as durable.
• Bound the overlay with byte backpressure and a hard page cap that drains before merging, stage size-only truncates, and drain the flusher on close.
• Add sqliteCommitMode actor config, SqliteDb sequence accessors, a database failure channel, sync-call fail-fast on JavaScript-owned transaction leases, and SQLite auto-rollback detection in the transaction coordinator.
• Expose db({ commitMode }), commitSeq(), flushedSeq(), waitForFlush(), flushError(), executeSyncRaw() with readonly, and a handle-form beginTransactionSync() in the TypeScript client, Drizzle wrapper, NAPI bindings, and runtime interfaces; WebAssembly and remote SQLite reject the mode at actor start.
• Fix the client action proxy so action.bind(...) is not dispatched as a nested remote action, and unlink the Actor Runtime Socket before joining its listener so a destroyed generation cannot expose its socket path.
• Add VFS, randomized, coordinator, client unit, and driver tests plus public docs.

Awaited mode is unchanged. The five fault::* compaction tests in rivet-depot-client fail identically on the base branch (in-process engine pubsub "invalid version: 0") and are unrelated.

[railway-app]

🚅 Deployed to the actors-pr-5676 environment in rivet-frontend

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