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

bench/event-storage.bench.ts

@@ -4,15 +4,19 @@ import { KVStorageInMemory } from '../src/kv-storage/kv-storage-in-memory.js'
 
 const events = new EventStorageKV(new KVStorageInMemory())
 const streams = events.child('streams')
-for (let i = 0; i < 10_000; i++) await streams.append('long', [{ i }])
+const ids = await streams.append(
+  'long',
+  Array.from({ length: 10_000 }, (_, i) => ({ i })),
+)
+const middle = ids[5000]
 let s = 0
 
 describe('events', () => {
-  bench('append 1 event (warm cache)', async () => {
+  bench('append 1 event', async () => {
     await streams.append(`s${s++ % 1000}`, [{ type: 'e' }])
   })
-  bench('read 100 events from version 5000 of 10k', async () => {
-    for await (const _ of streams.read('long', { from: 5000, limit: 100 })) {
+  bench('read 100 events from the middle of 10k', async () => {
+    for await (const _ of streams.read('long', { after: middle, limit: 100 })) {
     }
   })
   bench('read full 10k-event stream', async () => {