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/README.md

@@ -1,5 +1,5 @@
 ---
-status: draft
+status: stable
 last_updated: 2026-05-18
 ---
 
@@ -76,7 +76,7 @@ Uses UJSX as a direct dependency. Workflow templates are `UNode` trees. Renders
 - **graphology DAG** — structural analysis, cycle detection, topological sort via a `HostConfig`
 - **Reactive execution engine** — runtime workflow execution with signal-based status propagation
 
-See `docs/research/reconciler/05-flowgraph-host-configs.md` for the planned integration.
+See `../research/reconciler/05-flowgraph-host-configs.md` for the planned integration.
 
 ### Desktop UI (Spoke HUD)
 
@@ -118,11 +118,23 @@ last_updated: YYYY-MM-DD
 
 ADR documents use a separate `Status` field in their body: `Proposed`, `Accepted`, `Deprecated`, or `Superseded`. ADRs never revert from `Accepted`. Note that ADR frontmatter (`status: draft`) refers to the **document's editorial status** (is the writing complete?), while the body `Status` refers to the **decision's status** (is this decision finalized?). A `Proposed` decision can have a `draft` document; an `Accepted` decision has a `stable` document.
 
+## Error Handling Philosophy
+
+UJSX follows a tiered approach to errors:
+
+- **Programmer errors throw**. Invalid arguments (`h()` called without a type), missing required parameters, or `transform()` failing to find a matching rule — these are bugs that should surface immediately with a stack trace. Failing silently makes debugging harder.
+
+- **Operational conditions are no-ops with defined behavior**. `selectNode` returning `undefined` for invalid paths, `setNode` returning the node unchanged for unresolvable paths, `Fragment` with no children producing an empty array — these are not errors, they are well-defined boundary conditions. The caller is responsible for checking the result.
+
+- **Host errors propagate**. If `HostConfig.createInstance` throws, the error bubbles up through `mountNode` with no host involvement. UJSX does not catch or suppress host errors. The current implementation has no `handleError` hook on `HostConfig` — whether to add one is an open question (see [host-config.md](host-config.md) Open Question 4).
+
+- **Recovery is the caller's responsibility**. `ReactiveRoot.subscribe()` returns a dispose function; if the caller discards it, the effect leaks. `unmount()` is a stub. There is no automatic retry, circuit breaking, or error event emission for operational failures. These concerns belong to the host or consumer, not to the core library.
+
 ## References
 
-- UJSX research index: `docs/research/README.md`
-- Reconciler research: `docs/research/reconciler/`
-- SDD process: `docs/sdd_process.md`
+- UJSX research index: `../research/README.md`
+- Reconciler research: `../research/reconciler/`
+- SDD process: `../sdd_process.md`
 - Preact signals-core: `@preact/signals-core`
 - TypeBox: `@alkdev/typebox`
 - Taskgraph_ts architecture pattern: `/workspace/@alkdev/taskgraph_ts/docs/architecture/`