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

src/utils/errors.ts

@@ -1,15 +1,3 @@
-/** An optimistic-concurrency check failed (stale `expectedVersion` or a lost commit race). */
-export class ConcurrencyError extends Error {
-  constructor(
-    readonly stream: string,
-    readonly expectedVersion: number | 'any',
-    readonly actualVersion: number,
-  ) {
-    super(`Concurrency conflict on "${stream}": expected version ${expectedVersion}, actual ${actualVersion}`)
-    this.name = 'ConcurrencyError'
-  }
-}
-
 /** A key argument was passed but is `undefined`/`null`, empty, or not a string/number. */
 export class InvalidKeyError extends TypeError {
   constructor(message: string) {