feat(graph): cross-wing tunnels by shared topics (#1180)

When two wings have one or more confirmed TOPIC labels in common, the
miner now drops a symmetric tunnel between them at mine time so the
palace graph reflects shared themes (frameworks, vendors, recurring
concepts).

- llm_refine: TOPIC label routes to a dedicated `topics` bucket so the
  signal survives confirmation instead of getting collapsed into
  `uncertain` and dropped.
- entity_detector / project_scanner: bucket plumbed through the
  detection pipeline; `confirm_entities` returns confirmed topics
  alongside people/projects.
- miner.add_to_known_entities: optional `wing` parameter records the
  confirmed topics under `topics_by_wing` in
  `~/.mempalace/known_entities.json`. Wing names do NOT leak into the
  flat known-name set used by drawer-tagging.
- palace_graph: `compute_topic_tunnels` and `topic_tunnels_for_wing`
  create symmetric tunnels via the existing `create_tunnel` API so they
  share dedup and persistence with explicit tunnels.
- miner.mine: post-file-loop pass calls `topic_tunnels_for_wing` for
  the freshly-mined wing. Failures are logged but never abort the mine.
- config: `topic_tunnel_min_count` knob (env
  `MEMPALACE_TOPIC_TUNNEL_MIN_COUNT` or `~/.mempalace/config.json`),
  default 1.

Tests cover topic persistence through init->mine, tunnel creation when
wings share a topic, no tunnel below threshold, cross-wing tunnel
retrieval via `list_tunnels`, dedup on recompute, case-insensitive
overlap, and the end-to-end mine-time wiring.

Out of scope for this PR (called out in the PR body): manifest-
dependency overlap, per-topic allow/deny lists, search-result surfacing.

tests/test_entity_detector.py

@@ -235,13 +235,13 @@ def test_detect_entities_empty_files(tmp_path):
     f = tmp_path / "empty.txt"
     f.write_text("")
     result = detect_entities([f])
-    assert result == {"people": [], "projects": [], "uncertain": []}
+    assert result == {"people": [], "projects": [], "topics": [], "uncertain": []}
 
 
 def test_detect_entities_handles_missing_file(tmp_path):
     missing = tmp_path / "nonexistent.txt"
     result = detect_entities([missing])
-    assert result == {"people": [], "projects": [], "uncertain": []}
+    assert result == {"people": [], "projects": [], "topics": [], "uncertain": []}
 
 
 def test_detect_entities_respects_max_files(tmp_path):