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/keys.bench.ts

@@ -0,0 +1,26 @@
+import { bench, describe } from 'vitest'
+import { InMemoryKVDriver } from '../src/kv/memory.js'
+import { KVStorage } from '../src/kv/node.js'
+
+// 1k children, each with 100 descendants (100k keys under the node).
+const db = new KVStorage(new InMemoryKVDriver({ clone: false }))
+const node = db.child('n')
+const writes: [string, number][] = []
+for (let c = 0; c < 1000; c++) {
+  writes.push([`n/c${c}`, c])
+  for (let d = 0; d < 100; d++) writes.push([`n/c${c}/d${d}`, d])
+}
+await db.driver.setMany(writes)
+
+describe('keys() on 1k children x 100 descendants', () => {
+  bench('keys() skip-scan', async () => {
+    let n = 0
+    for await (const _ of node.keys()) n++
+    if (n !== 1000) throw new Error(`expected 1000, got ${n}`)
+  })
+  bench('naive full subtree scan (reference)', async () => {
+    let n = 0
+    for await (const _ of db.driver.scan({ gte: 'n/', lt: 'n0' })) n++
+    if (n !== 101_000) throw new Error(`expected 101000, got ${n}`)
+  })
+})