fix(searcher): guard against empty ChromaDB query results (#195) (#865)

Fixes #195.

When ChromaDB returns no documents (empty palace, or wing/room filter
that excludes everything), it returns the shape:

    {"documents": [], "metadatas": [], "distances": []}

Indexing `results["documents"][0]` blindly raises IndexError instead of
the expected 'no results' response. Affected: searcher.search(),
searcher.search_memories() (drawer + closet branches plus the
total_before_filter aggregate), and Layer3.search() / Layer3.search_raw().

Adds a tiny private helper `searcher._first_or_empty(results, key)` that
safely extracts the inner list, returning [] for any of: missing key,
empty outer list, [None], or [[]]. layers.py imports the same helper to
avoid duplicating the guard.

Tests: tests/test_empty_chromadb_results.py covers all observed shapes
plus a documentation-style test that pins the original IndexError so
future readers understand why the helper exists.

tests/test_empty_chromadb_results.py

@@ -0,0 +1,48 @@
+"""Regression tests for issue #195 — IndexError on empty ChromaDB results.
+
+Before the fix, `searcher.search()`, `searcher.search_memories()`, and
+`Layer3.search()` indexed `results["documents"][0]` without checking the
+outer list, so a query against an empty collection (or a wing/room
+filter that excluded everything) crashed with IndexError instead of
+returning a graceful "no results" response.
+"""
+
+import pytest
+
+from mempalace.searcher import _first_or_empty
+
+
+def test_first_or_empty_handles_empty_outer_list():
+    """The shape ChromaDB returns from an empty collection (issue #195)."""
+    results = {"documents": [], "metadatas": [], "distances": []}
+    assert _first_or_empty(results, "documents") == []
+    assert _first_or_empty(results, "metadatas") == []
+    assert _first_or_empty(results, "distances") == []
+
+
+def test_first_or_empty_handles_outer_with_empty_inner():
+    """ChromaDB also returns ``{"documents": [[]]}`` in some versions —
+    must yield [] either way."""
+    assert _first_or_empty({"documents": [[]]}, "documents") == []
+
+
+def test_first_or_empty_handles_missing_key():
+    assert _first_or_empty({}, "documents") == []
+
+
+def test_first_or_empty_handles_none_inner():
+    """``[None]`` (unusual but observed) must not blow up."""
+    assert _first_or_empty({"documents": [None]}, "documents") == []
+
+
+def test_first_or_empty_returns_inner_list_for_normal_result():
+    results = {"documents": [["a", "b", "c"]]}
+    assert _first_or_empty(results, "documents") == ["a", "b", "c"]
+
+
+def test_raw_indexing_still_raises_to_document_the_bug():
+    """Document the original failure mode so future readers understand
+    why _first_or_empty exists."""
+    results = {"documents": []}
+    with pytest.raises(IndexError):
+        _ = results["documents"][0]