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/reviews/001-architecture-gap-analysis.md

@@ -273,7 +273,7 @@ When addressing these issues, use this checklist to track progress:
 - [x] C-01: Fix README cross-reference link
 - [x] C-02: Add `CallEdgeAttrs` type alias to schema.md
 - [x] C-03: Resolve `OperationEdgeAttrs` vs `TypedEdgeAttrs` naming (renamed `TypedEdgeAttrs` → `OperationEdgeAttrs`)
-- [ ] C-04: Specify failure propagation semantics in reactive-execution.md
+- [x] C-04: Specify failure propagation semantics in reactive-execution.md (failure follows dependency edges, not structural scope; Conditionals as error boundaries; blockedByFailure computed; partial success for parallel branches)
 - [ ] C-05: Create FlowGraph public API document
 - [ ] C-06: Document `<Map>` component in workflow-templates.md
 - [ ] C-07: Specify `Conditional` else-branch behavior