Add KVStorageComposite, IndexedDB storage and SSE change streams

- Every KV storage emits change events (base class owns writeMany/removeMany/
  writeIfAbsent; implementations provide protected apply* methods)
- KVStorageHttpServer streams changes over SSE (GET /events) with a resumable
  history; KVStorageHttp.watch() subscribes and resumes via Last-Event-ID
- KVStorageIndexedDB for local persistence
- KVStorageComposite keeps replicas of a source in sync: local reads, resume
  instead of full sync, offline outbox replayed in order, echo tracking
- Values may contain bigint/Uint8Array (Extended JSON over the network)
- Vendor sse-session, event-emitter and extended-json (no runtime deps)
- Freeze child nodes so shadowed state on children fails loudly
- CI on Node 22, 24 and 26

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

CLAUDE.md

@@ -10,7 +10,7 @@ Layered TypeScript storage library: KV → Events → Documents. The design is i
 - `npm run typecheck`, `npm run build`, `npm run bench`
 - `npm run format` / `npm run format:check`: Prettier
 
-CI (`.gitea/workflows/ci.yml`, Gitea Actions) runs the format check, typecheck, tests and build on Node 20 and 22.
+CI (`.gitea/workflows/ci.yml`, Gitea Actions) runs the format check, typecheck, tests and build on Node 22, 24 and 26.
 
 ## Conventions
 
@@ -18,7 +18,11 @@ CI (`.gitea/workflows/ci.yml`, Gitea Actions) runs the format check, typecheck,
   primitives, and the user-facing API lives in the base class.
 - Classes over loose functions; TS `private`, not `#`; no overloads (use `param?` or tuple-union
   rest params); don't override base methods in subclasses.
-- No runtime dependencies. Tests live only in `test/`, mirroring `src/`.
+- **Never reassign a field after construction.** `child()` nodes are frozen `Object.create(root)`
+  views, so `this.x = ...` on a child throws. Keep mutable state in a `private readonly state`
+  object or in Maps (see `KVStorageComposite`, `KVStorageHttp`).
+- No runtime dependencies; third-party code is vendored (`src/utils/sse-session/`, `event-emitter.ts`,
+  `extended-json.ts`, with a header naming the source). Tests live only in `test/`, mirroring `src/`.
 - New KV/document implementations must pass `test/kv-storage/kv-storage.conformance.ts` /
   `test/document-storage/document-storage.suite.ts`.
 - Ask the owner before pushing.