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

test/kv-storage/node-server.ts

@@ -0,0 +1,24 @@
+import { createServer } from 'node:http'
+import type { AddressInfo } from 'node:net'
+
+/** Serves a `Request → Response` handler over a real socket (test helper). */
+export async function serve(handle: (request: Request) => Promise<Response>): Promise<{ url: string; close: () => Promise<void> }> {
+  const server = createServer(async (req, res) => {
+    const chunks: Buffer[] = []
+    for await (const chunk of req) chunks.push(chunk as Buffer)
+    const response = await handle(
+      new Request(`http://localhost${req.url}`, { method: req.method, body: req.method === 'POST' ? Buffer.concat(chunks) : undefined }),
+    )
+    res.writeHead(response.status, Object.fromEntries(response.headers))
+    if (!response.body) return void res.end()
+    const reader = response.body.getReader()
+    res.on('close', () => void reader.cancel().catch(() => {}))
+    for (let next = await reader.read(); !next.done; next = await reader.read()) res.write(next.value)
+    res.end()
+  })
+  await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve))
+  return {
+    url: `http://127.0.0.1:${(server.address() as AddressInfo).port}`,
+    close: () => new Promise((resolve) => (server.closeAllConnections(), server.close(() => resolve()))),
+  }
+}