address architecture review findings and add review document

Fixes from architecture review (4 critical, 10 warnings):

Critical:
- Fix selectNode/setNode docs to accurately describe prop-key
  navigation behavior including array support and prop-key writes
- Document RenderContext/Density exported types in host-config
- Resolve ADR dual status ambiguity with clarifying note in README
  (frontmatter status = editorial, body Status = decision)
- Effect types already addressed in prior commit

Warnings addressed:
- Add Fragment re-export note to jsx-runtime section in
  build-distribution
- Document childCtx/transformCtx helper functions in transforms.md
- Document render() accepting non-root UNode in host-config
- Add Value.Hash re-entrancy constraint to reconciler.md
- Add true-passthrough constraint and h('root') special case
  to element-factory constraints
- Add _idCounter bundling caveat note

Review document added at docs/reviews/architecture-review-2026-05-18.md
with full findings, source verification table, and recommendations.

docs/architecture/build-distribution.md

@@ -81,6 +81,8 @@ The `./jsx-runtime` export enables `jsxImportSource` configuration. When a consu
 
 TypeScript and other JSX transforms resolve `jsx`, `jsxs`, and `jsxDEV` from `@alkdev/ujsx/jsx-runtime`. All three are aliases for `h()` — UJSX does not distinguish between static children, dynamic children, or dev mode at the factory level.
 
+The `jsx-runtime` export also re-exports `Fragment` from `h.ts`. This means JSX consumers can import `{ Fragment }` from `@alkdev/ujsx/jsx-runtime` as well as from `@alkdev/ujsx/h` or the barrel export.
+
 ## Dependencies
 
 | Package | Version | Role | Hard/Peer? |