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/schema.md

@@ -97,6 +97,8 @@ type NodeStatus = Static<typeof NodeStatusEnum>;
 
 `NodeStatus` extends `CallStatus` with workflow-specific states (`idle`, `waiting`, `ready`, `skipped`) that have no call protocol equivalent. A node that is `waiting` has no call yet because its preconditions haven't been met.
 
+**Precondition semantics**: A predecessor in `completed` or `skipped` status satisfies a dependent's preconditions. A predecessor in `failed` or `aborted` status does NOT satisfy preconditions — it blocks the dependent and triggers failure propagation (the dependent transitions to `aborted`). This enables partial success: independent parallel branches continue running even when one branch fails.
+
 ### EdgeType
 
 The type of edge in a flowgraph. Matches the call graph storage schema's `edgeType` column: