refactor(search): address PR #750 round 6 review feedback

Closes out the remaining nits flagged in the round-6 review.

Critical:
- _verify_files contract comment now enumerates all None-return cases
  (404 + malformed PROPFIND XML) and documents the false-eviction
  trade-off; self-healing via re-indexing recovers
- int(r.id) cast at the SemanticSearchResult boundary now raises a
  TypeError with explicit doc_type/value context instead of bubbling
  up as an opaque "Search failed: ..." McpError

Design observations:
- nc_semantic_search_answer docstring documents the per-note
  round-trip cost from the post-verification race guard
- News verification latency hint added to configuration.md
- SemanticSearchResponse exposes verified_count + dropped_count so
  short result pages on high-ghost-density indexes are
  distinguishable from genuine scarcity. verify_search_results now
  returns (kept, dropped_count); production caller and tests updated

Minor:
- Comment clarifies the .get() fallback in verify_search_results is
  defensive only (run_verifier always populates the entry)
- Eviction task-group guard narrowed from except Exception to
  except RuntimeError (the only documented failure mode of
  TaskGroup.start_soon on a closed group)
- Indexer logs a warning when a deck_card task is missing
  board_id/stack_id, surfacing data-quality issues at index time
  rather than at verification time
- New unit test covers the news verifier's non-numeric-id fail-open
  path (one bad doc_id keeps the entire batch)

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

tests/integration/test_verify_on_read.py

@@ -58,9 +58,10 @@ async def test_verify_keeps_accessible_note(
     note_id = temporary_note["id"]
     results = [_result_for_note(note_id)]
 
-    kept = await verify_search_results(nc_client, results)
+    kept, dropped_count = await verify_search_results(nc_client, results)
 
     assert [r.id for r in kept] == [note_id]
+    assert dropped_count == 0
     spy_evict.assert_not_awaited()
 
 
@@ -96,9 +97,12 @@ async def test_verify_drops_deleted_note_and_schedules_eviction(
         await nc_client.notes.get_note(note_id)
     assert exc_info.value.response.status_code == 404
 
-    kept = await verify_search_results(nc_client, [_result_for_note(note_id)])
+    kept, dropped_count = await verify_search_results(
+        nc_client, [_result_for_note(note_id)]
+    )
 
     assert kept == [], "deleted note must not pass verification"
+    assert dropped_count == 1
     spy_evict.assert_awaited_once_with(note_id, "note", nc_client.username)
 
 
@@ -126,9 +130,10 @@ async def test_verify_mixed_accessible_and_deleted(
         _result_for_note(accessible_id),
         _result_for_note(ghost_id),
     ]
-    kept = await verify_search_results(nc_client, results)
+    kept, dropped_count = await verify_search_results(nc_client, results)
 
     assert [r.id for r in kept] == [accessible_id]
+    assert dropped_count == 1
     spy_evict.assert_awaited_once_with(ghost_id, "note", nc_client.username)
 
 
@@ -158,9 +163,10 @@ async def test_verify_dedupes_chunks_of_same_document(
         for i in range(3)
     ]
 
-    kept = await verify_search_results(nc_client, results)
+    kept, dropped_count = await verify_search_results(nc_client, results)
 
     # All three chunks kept (they're all from the same accessible note)
     assert len(kept) == 3
+    assert dropped_count == 0
     # ...but verification only fetched the note ONCE
     assert spy_get_note.await_count == 1