M8: HTTP driver (framework-agnostic handler + HttpKVDriver client) passing conformance end-to-end

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

DESIGN.md

@@ -566,6 +566,29 @@ Decisions taken where the brief was silent. Each is the simplest option consiste
 - **Type tests** live in `test/types.test-d.ts` and are enforced by `tsc --noEmit` (part of `npm test`),
   including the `@ts-expect-error` negative cases from section 7.
 
+### M8 — HTTP driver
+
+- **Extra endpoints** beyond the brief's list: `POST /deleteRange {gte, lt}` and
+  `GET /capabilities`. Unsupported optional operations return `501`, driver errors `500`, and unknown
+  routes `404`.
+- **Missing vs `null`**: `/getMany` returns `{ values, missing }`, where `missing` lists the indices of
+  absent keys, because `null` is a legitimate stored value.
+- **Codec scope**: the codec serialises whole request/response bodies and each NDJSON scan line
+  (`[key]` or `[key, value]`), so it must not emit newlines. The default is JSON.
+- **Capabilities on the client**: `HttpKVDriver` defines `setIfAbsent`/`batch` only when the server
+  supports them, so stores' feature detection (e.g. unique indexes requiring `setIfAbsent`, atomic
+  index writes using `batch`) behaves as it would locally. `HttpKVDriver.connect()` discovers them via
+  `/capabilities`. The constructor takes them explicitly (default: all). Without native
+  `deleteRange` the client falls back to `BaseKVDriver`'s chunked delete.
+- **Scan streaming**: the server pulls the driver iterator in chunks that grow from 16 to 256
+  entries, so short reads (skip-scan re-seeks) don't make it run far ahead. Cancelling the client
+  iterator cancels the response body, which calls `return()` on the server-side iterator.
+- **Optional `headers`** (e.g. auth) and an injectable `fetch` on the client; an optional `basePath` on
+  the handler.
+- The conformance suite runs end-to-end in-process (fetch → handler), over a real `node:http`
+  socket, and against a minimal server driver with discovered capabilities. A smoke test runs KV
+  indexes, events and unique-indexed documents over HTTP.
+
 <!-- decisions appended per milestone -->
 
 ## 13. Benchmark baselines