sched: add durable resume-attempt marker with store-backed identities (T09)

src/wf_artifacts/runs/models.py

@@ -127,4 +127,25 @@ class RunCheckpoint(BaseModel):
     sequence: int = Field(ge=1)
     reason: CheckpointReason
     state: PersistedRunState | VersionedCheckpointState
+    attempt_id: int | None = Field(
+        default=None,
+        ge=1,
+        description=(
+            "Store-backed resume-attempt identity that produced this result. "
+            "Recovery matches result to the active attempt: fresh results are "
+            "resumable, stale results fail closed without retry."
+        ),
+    )
     created_at: datetime
+
+
+class ResumeAttempt(BaseModel):
+    """Durable resume-attempt marker persisted before re-execution."""
+
+    model_config = ConfigDict(extra="forbid")
+
+    run_id: str = Field(pattern=RUN_ID_PATTERN)
+    attempt_id: int = Field(ge=1)
+    state: Literal["ACTIVE", "DONE"]
+    created_at: datetime
+    updated_at: datetime