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

bench/event-storage.bench.ts

@@ -0,0 +1,22 @@
+import { bench, describe } from 'vitest'
+import { EventStorageKV } from '../src/event-storage/event-storage-kv.js'
+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 }])
+let s = 0
+
+describe('events', () => {
+  bench('append 1 event (warm cache)', 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 full 10k-event stream', async () => {
+    for await (const _ of streams.read('long')) {
+    }
+  })
+})