Simplify: one abstract base class per layer

- BaseKVStorage / BaseEventStorage / BaseDocumentStorage define the API once;
  implementations provide only raw primitives (client and server share an API)
- Drop KV indexes, query planner, unique indexes; document stores take index fields
- Escape every char <= '/' so child keys stay contiguous; skip-scan is now simple
- Event-sourced documents = event storage + any document store for projections
- New folder layout (kv-storage, event-storage, document-storage, utils); tests only in test/

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

src/utils/errors.ts

@@ -0,0 +1,19 @@
+/** 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) {
+    super(message)
+    this.name = 'InvalidKeyError'
+  }
+}