feat(#194): Phase 1 — parentState / childThread in CAS nodes

- Protocol: StartNodePayload.parentState, StateNodePayload.childThread
- CAS: putStartNode refs include parentState, collectRefs includes childThread
- Parsing: legacy nodes without new fields default to null
- Engine + fork: all callers pass parentState: null / childThread: null
- Tests: 8 new cases for refs, parsing, collect-refs (+208 lines)

Phase 1 of #194 (Merkle Call Stack). Closes #195.

小橘 <xiaoju@shazhou.work>

packages/cli-workflow/__tests__/gc-cli.test.ts

@@ -45,8 +45,8 @@ describe("gc cli and garbageCollectCas", () => {
       {
         name: "demo",
         hash: bundleHash,
-
         depth: 0,
+        parentState: null,
       },
       promptHash,
     );
@@ -100,8 +100,8 @@ describe("gc cli and garbageCollectCas", () => {
       {
         name: "demo",
         hash: bundleHash,
-
         depth: 0,
+        parentState: null,
       },
       promptHash,
     );
@@ -135,8 +135,8 @@ describe("gc cli and garbageCollectCas", () => {
       {
         name: "demo",
         hash: bundleHash,
-
         depth: 0,
+        parentState: null,
       },
       promptHash,
     );