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/types.test-d.ts

@@ -84,6 +84,6 @@ describe('event and document types', () => {
       new DocumentStorageInMemory(),
     )
     expectTypeOf(docs.child('users').history('1')).toEqualTypeOf<AsyncIterable<StoredEvent<DocChange>>>()
-    expectTypeOf(docs.child('users').getAt('1', 3)).toEqualTypeOf<Promise<User | undefined>>()
+    expectTypeOf(docs.child('users').getAt('1', 'id')).toEqualTypeOf<Promise<User | undefined>>()
   })
 })