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

DESIGN.md

@@ -589,8 +589,58 @@ Decisions taken where the brief was silent. Each is the simplest option consiste
   socket, and against a minimal server driver with discovered capabilities. A smoke test runs KV
   indexes, events and unique-indexed documents over HTTP.
 
+### M9 — Benchmarks and in-memory driver performance
+
+- **The in-memory driver's sorted array was replaced by `ChunkedSortedMap`** (sorted chunks of ≤1024
+  keys, split in half on overflow; values in a `Map`). Profiling showed 59% of document-update time
+  in the sorted array's O(n) `splice` once the store held ~70k keys (each write adds commit, index
+  and claim keys). Inserts and deletes are now O(log n + 1024). Iteration is O(1) per step and
+  re-seeks after a mutation, which replaces the old version-counter revalidation in the driver.
+  The structure stays behind the `SortedMap` interface; a B-tree remains possible.
+- **Value cloning uses a hand-written JSON deep copy** instead of `structuredClone` (about 10× cheaper
+  per value), in the in-memory driver and the document layer. Values are JSON by contract (§10).
+
 <!-- decisions appended per milestone -->
 
 ## 13. Benchmark baselines
 
-_Filled in at milestone 9._
+`npm run bench` (Vitest bench), in-memory driver with default cloning, Node 22.22, Linux x64 cloud
+container (shared CPU, so expect ±10–20% run-to-run). Mean time per operation.
+
+**KV** (`bench/kv.bench.ts`): values are small objects.
+
+| Operation | 10k keys | 100k keys |
+|---|---|---|
+| `get` | 1.2 µs | 1.6 µs |
+| `set` (overwrite) | 1.7 µs | 2.0 µs |
+| `getMany` × 100 keys | 21 µs | 22 µs |
+
+**`keys()`** (`bench/keys.bench.ts`): node with 1 000 children, each with 100 descendants (101k keys).
+
+| Operation | Mean |
+|---|---|
+| `keys()` skip-scan (1 000 seeks) | 2.9 ms |
+| Reference: full subtree scan | 36 ms |
+
+**Events** (`bench/events.bench.ts`): single-event commits.
+
+| Operation | Mean |
+|---|---|
+| `append` 1 event (warm head cache) | 6.3 µs |
+| `read` 100 events from version 5 000 of 10 000 | 174 µs |
+| `read` full 10 000-event stream | 21 ms |
+| `version()` (reverse scan, `limit: 1`) | 3.9 µs |
+
+**Documents** (`bench/documents.bench.ts`): 10 000 docs, indexes on `age` and unique `email`.
+
+| Operation | Mean |
+|---|---|
+| `get` | 2.6 µs |
+| `update` (`$inc`, indexed node: projection read + append + batch) | 22 µs |
+| `findOne` by unique index (`email`) | 16 µs |
+| `find` indexed (`age = k`, ~100 hits) | 0.57 ms |
+| `find` full scan (`city = k`, ~200 hits of 10k) | 28 ms |
+
+For comparison, the first run with the sorted-array map and `structuredClone` measured: KV `get`
+3.7 µs, `getMany`×100 186 µs, `append` 34 µs, document `update` 110 µs, full scan 57 ms, and
+full stream read 62 ms.