stabilize architecture docs: address review findings and advance to stable

Critical fixes:
- Restructure pointers.md: move setNode prop-key writes section under
  its own heading (was incorrectly nested under selectNode)
- Add Context/Density/Direction/RenderContext documentation section
  to host-config.md (was only a brief constraint bullet)
- Advance all 5 ADRs from Status: Proposed → Accepted and frontmatter
  from status: draft → status: stable (decisions are driving implementation)
- Add error handling philosophy section to README

Warning/suggestion fixes:
- Add isUElement null check (node !== null) to schema.md discriminator table
- Add UjsxEnvelope convenience type documentation to events.md
- Add Direction Unicode arrow naming note to transforms.md
- Standardize all cross-references from absolute docs/research/ paths
  to relative ../research/ paths across all architecture docs
- Fix schema.md ADR references to use relative paths
- Reduce redundancy between transforms.md and host-config.md Direction notes
- Update all architecture doc frontmatter from draft → stable

Deferred:
- Performance model section (reconciler not yet built)
- Concepts/glossary document (low ROI at current scale)
- Line counts in source references (would date quickly)

docs/architecture/reconciler.md

@@ -1,5 +1,5 @@
 ---
-status: draft
+status: stable
 last_updated: 2026-05-18
 ---
 
@@ -209,7 +209,7 @@ Optimizations are applied in order:
 4. **Add `Value.Mutate`** (if function values are handled correctly) — preserve reference identity
 5. **Add `Value.Diff` for prop payloads** (optional, catch errors) — granular diff payloads for hosts that want them
 
-> See the research in `docs/research/reconciler/04-typebox-optimization-layer.md` for detailed analysis.
+> See the research in `../research/reconciler/04-typebox-optimization-layer.md` for detailed analysis.
 
 ## File Structure
 
@@ -256,11 +256,11 @@ Host implementations that currently only support mount-only rendering will start
 
 ## References
 
-- Reconciler research: `docs/research/reconciler/README.md`
-- Key field design: `docs/research/reconciler/00-KEY-FIELD-DESIGN.md`
-- Reactive → Host bridge: `docs/research/reconciler/01-reactive-host-bridge.md`
-- Children reconciliation: `docs/research/reconciler/02-key-based-children-reconciliation.md`
-- TypeBox optimization layer: `docs/research/reconciler/04-typebox-optimization-layer.md`
+- Reconciler research: `../research/reconciler/README.md`
+- Key field design: `../research/reconciler/00-KEY-FIELD-DESIGN.md`
+- Reactive → Host bridge: `../research/reconciler/01-reactive-host-bridge.md`
+- Children reconciliation: `../research/reconciler/02-key-based-children-reconciliation.md`
+- TypeBox optimization layer: `../research/reconciler/04-typebox-optimization-layer.md`
 - Lifecycle management: [lifecycle.md](lifecycle.md)
 - HostConfig interface: [host-config.md](host-config.md)
 - Reactive layer: [reactive-layer.md](reactive-layer.md)