refactor(search): address PR #750 round 7 review feedback

Round 7 raised 5 issues; this round addresses all of them and fixes
the underlying causes (not just the comments) where applicable so
they don't get re-flagged in future passes.

Critical:
- verified_count description in SemanticSearchResponse said "unique
  documents" but the value is len(verified_results), a chunk count.
  Description rewritten to accurately document chunk-level granularity
  AND explicitly call out the asymmetry with dropped_count (which
  counts unique (doc_id, doc_type) pairs).

- _verify_files false-eviction risk: the round-6 doc-only fix was
  re-flagged. Address at the source — widen WebDAVClient.get_file_info
  to raise HTTPStatusError on 404 (matching the rest of the client
  convention) and reserve None for the genuinely ambiguous
  malformed-PROPFIND case. _verify_files now keeps the result on None
  (cannot tell whether the file exists) and evicts only on a
  definitive HTTPStatusError 404. Tests updated; new test added for
  the malformed-XML keep-result path.

Non-critical:
- News verifier semaphore lifetime now explicitly documented: one
  slot held for one deduplicated fetch per search is the correct
  backpressure behaviour.

- Cross-reference comments in _verify_notes / _verify_deck_cards no
  longer claim "Mirrors X" pointing at functions defined later in
  the file; now use direction-neutral "parallel implementation in".

- accessible_by_type is mutated by concurrent run_verifier tasks; a
  comment explains why this is race-free under anyio's cooperative
  multitasking (distinct keys per task, no await between read and
  write) so a future reader doesn't add a redundant lock.

- Knock-on: tests/integration/test_rag.py wraps get_file_info in a
  try/except for the new contract.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/search/test_verification.py

@@ -317,9 +317,11 @@ async def test_verify_files_uses_path_from_metadata(mocker):
 
 
 @pytest.mark.unit
-async def test_verify_files_404_via_get_file_info_drops(mocker):
-    """get_file_info returns None on 404 — that's a definitive drop."""
-    webdav_client = SimpleNamespace(get_file_info=mocker.AsyncMock(return_value=None))
+async def test_verify_files_404_drops(mocker):
+    """get_file_info raising HTTPStatusError(404) is a definitive drop."""
+    webdav_client = SimpleNamespace(
+        get_file_info=mocker.AsyncMock(side_effect=_http_error(404))
+    )
     client = SimpleNamespace(webdav=webdav_client, username="alice")
 
     result = await _verify_files(
@@ -331,6 +333,27 @@ async def test_verify_files_404_via_get_file_info_drops(mocker):
     assert result == set()
 
 
+@pytest.mark.unit
+async def test_verify_files_malformed_propfind_keeps_result(mocker):
+    """get_file_info returning None means malformed PROPFIND — keep the result.
+
+    Per the contract change in webdav.py: ``None`` is now reserved for the
+    ambiguous "malformed XML" case. Real 404s raise HTTPStatusError. The
+    file verifier must NOT evict on the ambiguous case (we cannot tell
+    whether the file exists), only log a warning and keep the result.
+    """
+    webdav_client = SimpleNamespace(get_file_info=mocker.AsyncMock(return_value=None))
+    client = SimpleNamespace(webdav=webdav_client, username="alice")
+
+    result = await _verify_files(
+        client,
+        [_make_result(123, doc_type="file", metadata={"path": "brittle.txt"})],
+        _sem(),
+    )
+
+    assert result == {123}, "ambiguous None must keep result, not evict"
+
+
 @pytest.mark.unit
 async def test_verify_files_403_drops(mocker):
     """get_file_info raising HTTPStatusError(403) is a definitive drop."""