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/events.bench.ts

@@ -0,0 +1,27 @@
+import { bench, describe } from 'vitest'
+import { InMemoryKVDriver } from '../src/kv/memory.js'
+import { KVStorage } from '../src/kv/node.js'
+import { KVEventStorage } from '../src/events/kv.js'
+
+const events = new KVEventStorage(new KVStorage(new InMemoryKVDriver()))
+const streams = events.child('streams')
+const long = streams.child('long')
+for (let i = 0; i < 10_000; i++) await long.append([{ type: 'tick', i }])
+let s = 0
+
+describe('events', () => {
+  bench('append 1 event (warm head cache)', async () => {
+    await streams.append(`s${s++ % 1000}`, [{ type: 'e' }])
+  })
+  bench('read 100 events from version 5000 of 10k', async () => {
+    for await (const _ of long.read({ from: 5000, limit: 100 })) {
+    }
+  })
+  bench('read full 10k-event stream', async () => {
+    for await (const _ of long.read()) {
+    }
+  })
+  bench('version() (cold head lookup)', async () => {
+    await long.version()
+  })
+})