Fix: set cosine distance metadata on all collection creation sites

ChromaDB defaults HNSW index to L2 (Euclidean) distance, but
MemPalace scoring uses 1-distance which requires cosine (range 0-2).
Add metadata={"hnsw:space": "cosine"} to the 4 production and 3 test
call sites that were missing it.

Closes #218

mempalace/backends/chroma.py

@@ -35,8 +35,13 @@ def _fix_blob_seq_ids(palace_path: str):
                     continue
                 if not rows:
                     continue
-                updates = [(int.from_bytes(blob, byteorder="big"), rowid) for rowid, blob in rows]
-                conn.executemany(f"UPDATE {table} SET seq_id = ? WHERE rowid = ?", updates)
+                updates = [
+                    (int.from_bytes(blob, byteorder="big"), rowid)
+                    for rowid, blob in rows
+                ]
+                conn.executemany(
+                    f"UPDATE {table} SET seq_id = ? WHERE rowid = ?", updates
+                )
                 logger.info("Fixed %d BLOB seq_ids in %s", len(updates), table)
             conn.commit()
     except Exception:
@@ -71,7 +76,9 @@ class ChromaCollection(BaseCollection):
 class ChromaBackend:
     """Factory for MemPalace's default ChromaDB backend."""
 
-    def get_collection(self, palace_path: str, collection_name: str, create: bool = False):
+    def get_collection(
+        self, palace_path: str, collection_name: str, create: bool = False
+    ):
         if not create and not os.path.isdir(palace_path):
             raise FileNotFoundError(palace_path)
 
@@ -85,7 +92,9 @@ class ChromaBackend:
         _fix_blob_seq_ids(palace_path)
         client = chromadb.PersistentClient(path=palace_path)
         if create:
-            collection = client.get_or_create_collection(collection_name)
+            collection = client.get_or_create_collection(
+                collection_name, metadata={"hnsw:space": "cosine"}
+            )
         else:
             collection = client.get_collection(collection_name)
         return ChromaCollection(collection)