feat: add production-grade Web Worker data layer

[Copilot]

Implements a singleton, promise-based Web Worker data layer with cross-tab synchronisation, typed command dispatch, and a clean React hook API — no third-party abstractions.

Architecture

3-tier storage in the worker:

• Tier 1 — Map<string, unknown>: hot-path in-memory store, pre-warmed from IndexedDB on init
• Tier 2 — Dexie (arolariu-worker-state): durable persistence; written on SET/DELETE when persist=true
• Tier 3 — BroadcastChannel: fans out mutations to all other tabs; receivers update memory only (no re-persist, no re-broadcast → loop-safe)

New files

src/lib/workerTypes.ts

Shared discriminated-union contract. PayloadFor<T> and WorkerResultTypeMap give end-to-end generic inference — no any.

src/workers/coreWorker.ts

Stateful dedicated worker. Dispatch uses an async IIFE per message for full concurrency with per-request error isolation. Processor registry uses the Map constructor pattern (no eval, no dynamic code):

// Built-in operations
const processorRegistry = new Map<string, WorkerProcessor>([
  ["FILTER_BY_PREFIX", (data, state) => { /* … */ }],
  ["COUNT",            (_data, state) => state.size],
  ["KEYS",             (_data, state) => [...state.keys()]],
  ["CLEAR_ALL",        async () => { store.clear(); await db.state.clear(); /* broadcast */ }],
]);

src/workers/client.ts

Singleton WorkerClient (private constructor + getInstance()). In-flight requests are tracked in a Map<uuid, {resolve, reject}> — safe for arbitrary parallelism. Discriminates the READY signal on data.type === "READY" so TypeScript correctly narrows to WorkerResponse in the response path. onReady() uses queueMicrotask and returns an unsubscribe function.

src/hooks/useWorkerThread.ts

"use client" hook. All returned methods are useCallback-stabilised (empty deps) — safe in useEffect dependency arrays. Single reactive ready flag. WorkerClient.getInstance() is called inside useEffect only, ensuring no SSR execution.

function InvoiceCache() {
  const { ready, get, set, query, process } = useWorkerThread();

  useEffect(() => {
    if (!ready) return;
    void query("invoice:", 100).then(setData);
  }, [ready, query]);

  const count = async () => process("COUNT") as Promise<number>;
}