cold_clone: JIT hydrate skips missing/mismatched blobs (no crash)

Partial Tier B snapshots (where the producer didn't upload every
chunk, e.g. due to the corpus's BLOB-vs-TEXT-affinity skew) left
some NULL chunks without a corresponding blob in the bucket.
hydrate_doc_jit used to raise on the first NoSuchKey / hash
mismatch, which collapsed the whole JIT query. Now we skip the
offending chunk and let the query path see the same NULL content
it would see on a cold shard. Worst case: partial context, not a
crashed query.

arborist/cold_clone.py

@@ -414,26 +414,31 @@ def hydrate_doc_jit(
     if not rows:
         return 0
 
-    def _fetch(pair: tuple[int, str]) -> tuple[int, bytes]:
+    def _fetch(pair: tuple[int, str]):
         chunk_id, leaf_hash = pair
         key = f"{BLOB_PREFIX}/{leaf_hash[:2]}/{leaf_hash[2:]}"
-        body = backend.get(key)
+        try:
+            body = backend.get(key)
+        except Exception:
+            # Blob missing or unreadable — skip this chunk. The query
+            # path will see content still NULL and behave as it would
+            # for a cold shard (graceful degradation, not a crash).
+            return None
         if hash_leaf(body).hex() != leaf_hash:
-            raise ValueError(
-                f"chunk {chunk_id}: bucket blob hash mismatch (leaf_hash {leaf_hash[:12]})"
-            )
+            return None
         return chunk_id, body
 
     pairs = [(r[0], r[1]) for r in rows]
     n_workers = max(1, min(fetch_workers, len(pairs)))
     with ThreadPoolExecutor(max_workers=n_workers) as ex:
-        fetched = list(ex.map(_fetch, pairs))
-    with conn:
-        conn.executemany(
-            "UPDATE chunks SET content = ? WHERE chunk_id = ?",
-            [(body, cid) for cid, body in fetched],
-        )
-    return len(fetched)
+        results = [r for r in ex.map(_fetch, pairs) if r is not None]
+    if results:
+        with conn:
+            conn.executemany(
+                "UPDATE chunks SET content = ? WHERE chunk_id = ?",
+                [(body, cid) for cid, body in results],
+            )
+    return len(results)
 
 
 def fetch_chunk_jit(