feat(explorer): add node distance semantics to PathResponse (#472)

- Extend PathResponse with hop_count (len(path)-1) and distance_band
  ("direct"|"near"|"mid-range"|"distant") as first-class API fields
- Add classify_path_distance() to semantica/utils/helpers.py as the
  single source of truth for hop-count thresholds; both the route and
  the visualizer import from it, eliminating duplicate threshold logic
- Populate hop_count and distance_band in find_path route via
  classify_path_distance(); remove local _classify_distance() copy
- Add highlight_path: list[str] param to KGVisualizer.visualize_network;
  path edges rendered as a distance-aware orange trace (opacity and
  stroke width scale from direct→distant: 1.0/4px to 0.35/1.5px)
- Fix bidirectional edge lookup: only forward pairs (A→B) along the
  path are added to path_edge_set; reverse back-edges in directed
  graphs are no longer incorrectly highlighted
- Add logger.warning when highlight_path contains node IDs absent from
  the layout position map, surfacing silent no-op mismatches
- Extend frontend PathResponse type in GraphInspectorPanel.tsx and
  GraphWorkspaceShell.tsx with hop_count: number and distance_band
  literal union to match the updated API contract
- Add 10 new tests: 2 API-level and 8 unit tests covering all four
  band boundaries (0, 1, 2, 3, 4, 6, 7, 20 hops); 104 explorer tests
  pass, 0 failures introduced
- Update CHANGELOG.md

explorer/src/workspaces/GraphWorkspace/GraphWorkspaceShell.tsx

@@ -33,6 +33,8 @@ type LinkPrediction = {
 type PathResponse = {
   path: GraphPath;
   total_weight: number;
+  hop_count: number;
+  distance_band: "direct" | "near" | "mid-range" | "distant";
 };
 
 type TemporalBounds = {