docs: record the async-route (choo#653) design as a Phase 3 deliverable

Deliberately deferred so the SSR half (toStream) ships together with the
browser half — the missing server story is what stalled the original PR.
API shape goes to the RFC.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014NgfSjHE11oFpoSnLVKLXd

docs/v8.md

@@ -47,7 +47,14 @@ everything new lives under `packages/`.
 - [ ] Phase 2 (remaining): adoption-style hydration with mismatch
       warnings, Playwright suite in a real browser, benchmarks vs
       nanohtml v1 / µhtml
-- [ ] Phase 3: v8 wiring (`toStream`, devtools, codemod)
+- [ ] Phase 3: v8 wiring (`toStream`, devtools, codemod). Includes the
+      answer to choojs/choo#653 (async/lazy routes): route handlers may
+      resolve lazily via native import(); browser renders previous tree
+      or a loading view until resolution, server awaits inside toStream
+      (toString stays sync and fails loudly). Deliberately deferred until
+      toStream exists so the SSR half ships with the browser half — the
+      missing server story is what stalled #653. API shape (thenable
+      handler vs lazy() wrapper vs loader-view option) goes to the RFC.
 - [ ] Phase 4: bankai v10 (Vite 8/Rolldown shell, SSR middleware,
       103 Early Hints, service worker, precompression)
 - [ ] Phase 5: docs, examples, launch