M9: benchmarks, chunked sorted map and JSON clone for the in-memory driver

Profiling showed document updates dominated by O(n) sorted-array splices; the
chunked map makes inserts O(log n + chunk). Baselines recorded in DESIGN.md.

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

bench/kv.bench.ts

@@ -0,0 +1,29 @@
+import { bench, describe } from 'vitest'
+import { InMemoryKVDriver } from '../src/kv/memory.js'
+import { KVStorage } from '../src/kv/node.js'
+
+async function populated(n: number) {
+  const db = new KVStorage(new InMemoryKVDriver())
+  const users = db.child('users')
+  const batch: [string, { name: string; n: number }][] = []
+  for (let i = 0; i < n; i++) batch.push([`u${i}`, { name: `user ${i}`, n: i }])
+  await users.setMany(batch)
+  return users
+}
+
+for (const n of [10_000, 100_000]) {
+  const users = await populated(n)
+  const keys = Array.from({ length: 100 }, (_, i) => `u${(i * 7919) % n}`)
+  let i = 0
+  describe(`KV @ ${n / 1000}k keys`, () => {
+    bench('get', async () => {
+      await users.get(`u${i++ % n}`)
+    })
+    bench('set (overwrite)', async () => {
+      await users.set(`u${i++ % n}`, { name: 'x', n: i })
+    })
+    bench('getMany x100', async () => {
+      await users.getMany(keys)
+    })
+  })
+}