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/transforms.md

@@ -93,6 +93,18 @@ The "first match wins" semantics mean that priority and registration order resol
 
 Maps `transform()` over an array, passing each node's index as `ctx.index`. This is a convenience for transforming child lists — it preserves the ancestor stack from `ctx` without requiring callers to manage index tracking.
 
+## Helper Functions
+
+The transform module exports two context factory functions used alongside `TransformRegistry`:
+
+### `ctx<A>(direction, ancestors?, index?, metadata?)`
+
+Creates a `TransformContext` from its arguments. The `direction` parameter is required; the rest default to `[]`, `0`, and `{}` respectively. Exported as `transformCtx` from the barrel (`@alkdev/ujsx/transform`) to avoid name collision with React's `ctx` naming.
+
+### `childCtx<A>(parent, ctx, index)`
+
+Creates a new `TransformContext` with `parent` pushed onto the ancestors stack and `index` set. This is the standard way to descend into a child node during transformation — it preserves the direction and metadata from the parent context while updating traversal state.
+
 ## Direction Definitions
 
 The six directions pair into three bi-directional channels: