feat: implement node status signal management with computed preconditions and blockedByFailure

- Add computePreconditions and computeBlockedByFailure functions to node-status.ts
- Add registerStartEffect and registerAbortEffect for automatic state transitions
- Start effect: idle/waiting -> ready when preconditions met
- Abort effect: idle/waiting -> aborted when blockedByFailure true
- Refactor WorkflowReactiveRoot to use node-status.ts functions
- Root nodes auto-transition from idle to ready (no predecessors = preconditions true)
- Add AbortEffectOptions with abortDependents policy support
- Add comprehensive unit tests for all precondition and failure isolation scenarios

src/reactive/index.ts

@@ -10,3 +10,12 @@ export {
   type EventLogProjection,
   type AggregateStatus,
 } from "./workflow.js";
+
+export {
+  computePreconditions,
+  computeBlockedByFailure,
+  registerStartEffect,
+  registerAbortEffect,
+  type NodeStatusContext,
+  type AbortEffectOptions,
+} from "./node-status.js";