fix(runtime): fail closed on offline artifact misses

src/k1link/artifact_gateway.py

@@ -16,7 +16,7 @@ from collections.abc import Iterator, Mapping, Sequence
 from contextlib import contextmanager, suppress
 from dataclasses import dataclass
 from pathlib import Path
-from typing import Any, Final
+from typing import Any, Final, Literal
 from uuid import uuid4
 
 from k1link.artifacts import utc_now_iso
@@ -98,6 +98,12 @@ class ArtifactCacheStatus:
     free_space_reserve_bytes: int
 
 
+@dataclass(frozen=True, slots=True)
+class ArtifactGatewayStatus:
+    central_status: Literal["ready", "unavailable", "invalid"]
+    cache: ArtifactCacheStatus
+
+
 class CentralArtifactStore:
     """Immutable SHA-256 objects/manifests plus atomically replaceable named refs."""
 
@@ -617,6 +623,22 @@ class ArtifactGateway:
         self.store = store
         self.cache = cache
 
+    def status(self) -> ArtifactGatewayStatus:
+        """Return a lightweight readiness snapshot without resolving an artifact."""
+
+        try:
+            self.store._require_root()
+        except ArtifactStoreUnavailable:
+            central_status: Literal["ready", "unavailable", "invalid"] = "unavailable"
+        except ArtifactIntegrityError:
+            central_status = "invalid"
+        else:
+            central_status = "ready"
+        return ArtifactGatewayStatus(
+            central_status=central_status,
+            cache=self.cache.status(),
+        )
+
     def publish(
         self,
         *,