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/node-status.ts

@@ -1 +1,81 @@
-export {};
+import { effect } from "@preact/signals-core";
+import type { Signal, ReadonlySignal } from "@preact/signals-core";
+import type { NodeStatus } from "../schema/enums.js";
+
+const TERMINAL_STATUSES: Set<NodeStatus> = new Set([
+  "completed",
+  "failed",
+  "aborted",
+  "skipped",
+]);
+
+export interface NodeStatusContext {
+  statusMap: Map<string, Signal<NodeStatus>>;
+  predecessors: string[];
+}
+
+export function computePreconditions(
+  _nodeKey: string,
+  ctx: NodeStatusContext,
+): boolean {
+  if (ctx.predecessors.length === 0) return true;
+  return ctx.predecessors.every((pred: string) => {
+    const predStatus = ctx.statusMap.get(pred);
+    if (!predStatus) return false;
+    return predStatus.value === "completed" || predStatus.value === "skipped";
+  });
+}
+
+export function computeBlockedByFailure(
+  _nodeKey: string,
+  ctx: NodeStatusContext,
+): boolean {
+  return ctx.predecessors.some((pred: string) => {
+    const predStatus = ctx.statusMap.get(pred);
+    if (!predStatus) return false;
+    return predStatus.value === "failed" || predStatus.value === "aborted";
+  });
+}
+
+export function registerStartEffect(
+  status: Signal<NodeStatus>,
+  preconditions: ReadonlySignal<boolean>,
+  effectDisposers: (() => void)[],
+): void {
+  const disposer = effect(() => {
+    if (preconditions.value) {
+      const current = status.value;
+      if (current === "idle" || current === "waiting") {
+        status.value = "ready";
+      }
+    }
+  });
+  effectDisposers.push(disposer);
+}
+
+export interface AbortEffectOptions {
+  abortDependents?: boolean;
+}
+
+export function registerAbortEffect(
+  status: Signal<NodeStatus>,
+  blockedByFailure: ReadonlySignal<boolean>,
+  effectDisposers: (() => void)[],
+  options?: AbortEffectOptions,
+): void {
+  const disposer = effect(() => {
+    if (blockedByFailure.value) {
+      const current = status.value;
+      if (options?.abortDependents) {
+        if (!TERMINAL_STATUSES.has(current)) {
+          status.value = "aborted";
+        }
+      } else {
+        if (current === "idle" || current === "waiting") {
+          status.value = "aborted";
+        }
+      }
+    }
+  });
+  effectDisposers.push(disposer);
+}