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/element-factory.md

@@ -1,5 +1,5 @@
 ---
-status: draft
+status: stable
 last_updated: 2026-05-18
 ---
 
@@ -155,4 +155,4 @@ This is documented in the schema architecture ([schema.md](schema.md) — Known
 - Schema architecture: `docs/architecture/schema.md`
 - Key field ADR: [decisions/004-key-as-first-class-field.md](decisions/004-key-as-first-class-field.md)
 - Reconciler architecture: [reconciler.md](reconciler.md)
-- Key-based reconciliation research: `docs/research/reconciler/02-key-based-children-reconciliation.md`
+- Key-based reconciliation research: `../research/reconciler/02-key-based-children-reconciliation.md`