feat: v0.5.0 feature round - measurement, view, tree, section, export upgrades

Measurement / interaction:
- Circle-center snap in Snap(): click near a circular edge to snap to its
  center (hole-to-hole pitch measuring); nearest of vertex vs circle wins
- Esc cancels pending multi-step measurement, then exits mode, then gizmo;
  hotkeys P/D/E/F/C/A/M toggle measure modes (ignored while typing)
- Volume/centroid via mesh signed volume (open-shell detection rejects
  unreliable results); tree context menu adds it as a measurement

View / UI:
- Standard views (Iso/Front/Top/Right, Z-up) + orthographic toggle;
  gizmo overlay camera now switches type to match main camera
- GridSplitters for tree/measurement panels; node tooltip shows AABB LxWxH
- Tree name filter (matches + ancestors + subtree, auto-expand);
  context menu: isolate node / invert visibility / show all
- Settings persistence (window bounds, unit, 10-file MRU) in
  %LOCALAPPDATA%/STPViewer/settings.json; recent-files dropdown

Files / section:
- Interference accepts >=2 visible files, checks all pairs
- Export STEP: visible Solids/Shells at current aligned positions via
  CADability ExportStep; SmokeTest --export-test round-trip verifies
- Custom section plane by picking 3 points (axis combo 3-pt); plane
  position generalized to AABB-corner projection; numeric position input
- Bump version to 0.5.0

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/SmokeTest/Program.cs

@@ -35,6 +35,9 @@ if (args[0] == "--align-test")
 if (args[0] == "--bbox")
     return SmokeTest.BBoxDump.Run(args[1]);
 
+if (args[0] == "--export-test")
+    return SmokeTest.ExportTest.Run(args[1], args[2]) == 0 ? 0 : 2;
+
 var service = new StepImportService { Progress = s => Console.WriteLine($"  [階段] {s}") };
 int failed = 0;