fix: guard None metadata/doc in tool_check_duplicate and Layer1/Layer2

Chroma 1.5.x can return ``None`` inside the ``metadatas`` / ``documents``
lists of a query/get result for partially-flushed rows. The codebase
already has a systemic None-guard pattern (merged #999, #1013, #1019)
but three call sites were still unguarded:

* ``mcp_server.tool_check_duplicate`` (``mcp_server.py:487-488``) —
  ``meta = results["metadatas"][0][i]`` followed by ``meta.get(...)``
  raises ``AttributeError: 'NoneType' object has no attribute 'get'``.
  The broad ``except Exception`` wrapper (line 504) swallows it and
  returns an uninformative ``"Duplicate check failed"``.

* ``layers.Layer1.generate`` (``layers.py:126``) — iterates
  ``zip(docs, metas)`` and calls ``meta.get(key)`` in the importance
  loop. A single None metadata blows up the entire wake-up render.

* ``layers.Layer2.retrieve`` (``layers.py:224``) — same pattern, same
  crash path for the on-demand render.

Apply the same ``meta = meta or {}`` / ``doc = doc or ""`` idiom used
by the merged guards in the search path. Three-line additions, no
behaviour change on well-formed results.

Tests added:

* ``test_check_duplicate_handles_none_metadata`` — mocks the collection
  query to return ``None`` for one metadata and document, asserts the
  call does not crash and the sentinel-rendered entry has wing/room "?"
  and empty content.
* ``test_layer1_handles_none_metadata`` / ``_handles_none_document``
* ``test_layer2_handles_none_metadata``

Relationship to other open PRs:

* **#1019** guarded ``searcher.py`` loops. This PR extends the same
  guard to the three call sites #1019 did not touch.
* **#979** fixed ``tool_check_duplicate`` negative similarity but left
  the None-metadata path unguarded.
* Does not overlap **#1013** (``Layer3.search_raw``) or **#999**.

mempalace/mcp_server.py

@@ -484,8 +484,10 @@ def tool_check_duplicate(content: str, threshold: float = 0.9):
                 dist = results["distances"][0][i]
                 similarity = round(1 - dist, 3)
                 if similarity >= threshold:
-                    meta = results["metadatas"][0][i]
-                    doc = results["documents"][0][i]
+                    # Chroma 1.5.x can return None for partially-flushed rows;
+                    # coerce to empty sentinels so downstream .get() is safe.
+                    meta = results["metadatas"][0][i] or {}
+                    doc = results["documents"][0][i] or ""
                     duplicates.append(
                         {
                             "id": drawer_id,