Plain operations: last write wins, hash-keyed events, simpler composite

- KV: one write primitive, write(changes) with { key, value? }; writeIfAbsent removed.
  The server executes exactly what clients send and never reads values, so they can be
  encrypted end to end. Adds setFields/getFields on the base class.
- HTTP: no server history or event ids; SSE relays every change (own writes included) and
  the client emits resync when a dropped stream reconnects.
- Composite: writes apply locally, then go to the source (undone locally if refused);
  pushed changes are applied and announced only where they differ from local values;
  offline writes go to a queue store (any KV storage); sync() = replay, diff, replay.
  Removes KVReplicaSet, KVOutbox, KeyCounter, SerialQueue and RetryTimer.
- Events: id = <time>_<sha256(time + canonical data)>, so concurrent appends never
  collide and retries can't duplicate; no versions or ConcurrencyError.
- Event-sourced documents: read, resolve, append, apply; catchUp folds in foreign events;
  getAt takes an event id.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

test/kv-storage/kv-storage-http.test.ts

@@ -30,7 +30,7 @@ describe('over a real socket', () => {
 
   it('stops a long scan early', async () => {
     const db = fresh()
-    await db.writeMany(Array.from({ length: 5000 }, (_, i) => [`k${String(i).padStart(5, '0')}`, i] as [string, number]))
+    await db.writeMany(Array.from({ length: 5000 }, (_, i) => ({ key: `k${String(i).padStart(5, '0')}`, value: i })))
     const seen: string[] = []
     for await (const [key] of db.scan()) if (seen.push(key) === 3) break
     expect(seen).toEqual(['k00000', 'k00001', 'k00002'])