specify failure propagation semantics (C-04)

Key design decisions:
- Failure follows dependency edges, not structural scope
- Parallel branches are independent: failure in one branch
  doesn't cancel sibling branches
- blockedByFailure computed signal detects failed/aborted predecessors
- Conditionals serve as error boundaries (caught failures redirect
  to else branch, uncaught failures cascade)
- aborted nodes don't satisfy preconditions; skipped nodes do
- abortAll() for systemic failures (provider outage, auth failure)

Changes:
- reactive-execution.md: new Failure Propagation section with
  sequential/parallel/join/conditional examples, blockedByFailure
  effect, partial success model
- host-configs.md: add blockedByFailure to WorkflowNode, update
  status propagation effects, replace cascadeAbort with abortAll
- schema.md: document precondition semantics for NodeStatus
- build-distribution.md + README.md: add blockedByFailure to
  node-status.ts comments
- review checklist: mark C-04 resolved

docs/architecture/build-distribution.md

@@ -36,7 +36,7 @@ Package structure, exports map, dependencies, and platform targets.
 │   │   └── index.ts
 │   ├── reactive/
 │   │   ├── workflow.ts         # WorkflowReactiveRoot (signal-backed execution)
-│   │   ├── node-status.ts      # Signal<NodeStatus>, computed preconditions
+│   │   ├── node-status.ts      # Signal<NodeStatus>, computed preconditions, computed blockedByFailure
 │   │   └── index.ts
 │   ├── analysis/
 │   │   ├── type-compat.ts      # typeCompat, buildTypeEdges, analyzeTypeCompat