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-in-memory.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import fc from 'fast-check'
 import { KVStorageInMemory } from '../../src/kv-storage/kv-storage-in-memory.js'
-import type { BaseKVStorage } from '../../src/kv-storage/base-kv-storage.js'
+import type { BaseKVStorage, KVChange } from '../../src/kv-storage/base-kv-storage.js'
 import { collect, KVStorageCounting } from '../helpers.js'
 import { runKVStorageConformance } from './kv-storage.conformance.js'
 
@@ -16,14 +16,14 @@ describe('costs', () => {
     await users.get('1')
     await users.child('1').get()
     await users.delete('1')
-    expect(db.calls).toEqual({ writeMany: 1, readMany: 2, removeMany: 1 })
+    expect(db.calls).toEqual({ writeMany: 2, readMany: 2 })
   })
 
   it('keys() seeks once per child with descendants, not once per key', async () => {
     const db = new KVStorageCounting()
-    const writes: [string, number][] = []
-    for (let c = 0; c < 50; c++) for (let d = 0; d < 20; d++) writes.push([`n/c${c}/d${d}`, d])
-    for (let c = 0; c < 50; c++) writes.push([`n/leaf${c}`, c])
+    const writes: KVChange[] = []
+    for (let c = 0; c < 50; c++) for (let d = 0; d < 20; d++) writes.push({ key: `n/c${c}/d${d}`, value: d })
+    for (let c = 0; c < 50; c++) writes.push({ key: `n/leaf${c}`, value: c })
     await db.writeMany(writes)
     db.reset()
     expect(await collect(db.child('n').keys())).toHaveLength(100)