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

tests/conftest.py

@@ -101,7 +101,9 @@ def config(tmp_dir, palace_path):
 def collection(palace_path):
     """A ChromaDB collection pre-seeded in the temp palace."""
     client = chromadb.PersistentClient(path=palace_path)
-    col = client.get_or_create_collection("mempalace_drawers")
+    col = client.get_or_create_collection(
+        "mempalace_drawers", metadata={"hnsw:space": "cosine"}
+    )
     yield col
     client.delete_collection("mempalace_drawers")
     del client
@@ -185,7 +187,9 @@ def seeded_kg(kg):
     kg.add_triple("Alice", "parent_of", "Max", valid_from="2015-04-01")
     kg.add_triple("Max", "does", "swimming", valid_from="2025-01-01")
     kg.add_triple("Max", "does", "chess", valid_from="2024-06-01")
-    kg.add_triple("Alice", "works_at", "Acme Corp", valid_from="2020-01-01", valid_to="2024-12-31")
+    kg.add_triple(
+        "Alice", "works_at", "Acme Corp", valid_from="2020-01-01", valid_to="2024-12-31"
+    )
     kg.add_triple("Alice", "works_at", "NewCo", valid_from="2025-01-01")
 
     return kg