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/client/test_webdav.py

@@ -164,8 +164,14 @@ async def test_get_file_info_returns_file_details(mocker):
 
 
 @pytest.mark.unit
-async def test_get_file_info_returns_none_for_missing_file(mocker):
-    """Test that get_file_info returns None for missing files."""
+async def test_get_file_info_raises_on_404(mocker):
+    """get_file_info now raises HTTPStatusError on 404 (was: returned None).
+
+    The contract was widened so verify-on-read can distinguish a definitive
+    404 from an ambiguous malformed-PROPFIND response. Callers that want
+    "absent → None" semantics should catch HTTPStatusError and check the
+    status code themselves.
+    """
     from httpx import HTTPStatusError, Response
 
     mock_http_client = AsyncMock()
@@ -180,11 +186,10 @@ async def test_get_file_info_returns_none_for_missing_file(mocker):
         )
     )
 
-    # Call get_file_info
-    result = await client.get_file_info("nonexistent.pdf")
+    with pytest.raises(HTTPStatusError) as exc_info:
+        await client.get_file_info("nonexistent.pdf")
 
-    # Verify result is None
-    assert result is None
+    assert exc_info.value.response.status_code == 404
 
 
 @pytest.mark.unit