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/ExportTest.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using CADability;
+using CADability.GeoObject;
+using STPViewer.Services;
+using IOPath = System.IO.Path; // CADability.GeoObject.Path（曲線）撞名
+
+namespace SmokeTest;
+
+/// <summary>
+/// STEP 匯出往返驗證（v0.5.0 匯出 STEP 功能的無 UI 驗證）：
+/// 匯入來源 STEP → 走與 MainViewModel.ExportStepFile 相同路徑寫出新檔 → 重新匯入比對實體數。
+/// </summary>
+public static class ExportTest
+{
+    public static int Run(string sourcePath, string outPath)
+    {
+        Console.WriteLine($"=== ExportTest: {IOPath.GetFileName(sourcePath)} → {IOPath.GetFileName(outPath)} ===");
+        ImportedFileData data = new StepImportService().Import(sourcePath);
+        var geos = Collect(data.Root).Where(g => g is Solid or Shell).ToList();
+        Console.WriteLine($"  來源 Solid/Shell 物件 = {geos.Count}（Solids = {data.Root.SolidCount}）");
+
+        Project project = Project.CreateSimpleProject();
+        Model model = project.GetActiveModel();
+        foreach (IGeoObject g in geos) model.Add(g);
+        new ExportStep().WriteToFile(outPath, project);
+        var fi = new FileInfo(outPath);
+        Console.WriteLine($"  已寫出（{fi.Length:N0} bytes）");
+        if (fi.Length < 100)
+        {
+            Console.WriteLine("ExportTest: FAIL（輸出檔近乎空白）");
+            return 1;
+        }
+
+        ImportedFileData back = new StepImportService().Import(outPath);
+        Console.WriteLine($"  回讀 Solids = {back.Root.SolidCount}, Faces = {back.Root.FaceCount}");
+        bool ok = back.Root.SolidCount == data.Root.SolidCount && back.Root.FaceCount > 0;
+        Console.WriteLine(ok
+            ? "ExportTest: PASS（實體數一致）"
+            : $"ExportTest: FAIL（來源 Solids {data.Root.SolidCount} ↔ 回讀 {back.Root.SolidCount}）");
+        return ok ? 0 : 1;
+    }
+
+    private static IEnumerable<IGeoObject> Collect(ImportedNode n)
+    {
+        foreach (IGeoObject g in n.SourceGeos) yield return g;
+        foreach (ImportedNode c in n.Children)
+            foreach (IGeoObject g in Collect(c))
+                yield return g;
+    }
+}