test(恢复夹具): 固化四类可重放检查点快照

新增 Direction、Structure、CandidatePortfolio 与 Root ACCEPT 的确定性 fixture，记录输入快照、任务树、Artifact digest 和阶段边界，为离线重放后半程提供稳定数据。

script_build_host/tests/fixtures/recovery_checkpoints.json

@@ -0,0 +1,25 @@
+{
+  "schema_version": "script-build-checkpoint-fixtures/v1",
+  "checkpoints": {
+    "direction": {
+      "boundary": "PHASE_ONE_CAPABILITY_BOUNDARY",
+      "accepted_task_kinds": ["decode-retrieval", "direction"],
+      "input_snapshot_digest": "sha256:1111111111111111111111111111111111111111111111111111111111111111"
+    },
+    "structure": {
+      "boundary": "STRUCTURE_ACCEPTED",
+      "accepted_task_kinds": ["direction", "structure"],
+      "input_snapshot_digest": "sha256:2222222222222222222222222222222222222222222222222222222222222222"
+    },
+    "portfolio": {
+      "boundary": "PHASE_TWO_CANDIDATE_PORTFOLIO_READY",
+      "accepted_task_kinds": ["direction", "structure", "paragraph", "compose", "candidate-portfolio"],
+      "input_snapshot_digest": "sha256:3333333333333333333333333333333333333333333333333333333333333333"
+    },
+    "root": {
+      "boundary": "ROOT_ACCEPTED",
+      "accepted_task_kinds": ["direction", "candidate-portfolio", "root-delivery"],
+      "input_snapshot_digest": "sha256:4444444444444444444444444444444444444444444444444444444444444444"
+    }
+  }
+}