fix: guard Layer3.search_raw against None doc/meta from ChromaDB (#1011)

Same class of bug as #1007: ChromaDB's query() can return None in the
documents and metadatas arrays when a drawer's HNSW vector entry exists
but its metadata/document rows haven't been materialized. The code in
Layer3.search_raw (mempalace/layers.py) calls meta.get("wing", ...),
meta.get("room", ...), meta.get("source_file", ...) directly without
null safety, so it raises:

  AttributeError: 'NoneType' object has no attribute 'get'

Two-line defensive coercion matching the pattern in #1009 /
PR #999 for searcher.py: meta = meta or {}, doc = doc or "".
The hit still appears with its real distance; source/wing/room
fall back to their fallback values where the metadata row is missing.

Frequently hit on chromadb 1.5.x (root cause #1006). Even after the
chromadb floor lands (#1010), partial-state results remain possible
during interrupted mines and schema upgrade boundaries, so the guard
is worth having on its own.

Fixes #1011.

mempalace/layers.py

@@ -281,6 +281,8 @@ class Layer3:
 
         lines = [f'## L3 — SEARCH RESULTS for "{query}"']
         for i, (doc, meta, dist) in enumerate(zip(docs, metas, dists), 1):
+            meta = meta or {}
+            doc = doc or ""
             similarity = round(1 - dist, 3)
             wing_name = meta.get("wing", "?")
             room_name = meta.get("room", "?")
@@ -327,6 +329,13 @@ class Layer3:
             _first_or_empty(results, "metadatas"),
             _first_or_empty(results, "distances"),
         ):
+            # ChromaDB may return None for doc/meta when a drawer's HNSW entry
+            # exists but its metadata/document rows haven't been materialized
+            # (partial-flush states, mid-delete, schema upgrade boundaries).
+            # Degrade gracefully — the hit still appears with real distance;
+            # storage fields show their fallback where content is missing.
+            meta = meta or {}
+            doc = doc or ""
             hits.append(
                 {
                     "text": doc,