chore: Add --bbox SmokeTest diagnostic and v0.3.1 release notes

BBoxDump prints per-leaf world-space bounding boxes (to work out each
connector's mating-face axis); wired as `SmokeTest --bbox <file>`. Also tracks
the previously untracked v0.3.1 release notes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/SmokeTest/BBoxDump.cs

@@ -0,0 +1,28 @@
+// One-shot diagnostic: print per-leaf world-space bounding boxes so we can
+// figure out each connector's mating-face normal (axis the guide posts / holes point).
+using STPViewer.Services;
+
+namespace SmokeTest;
+
+internal static class BBoxDump
+{
+    public static int Run(string path)
+    {
+        var svc = new StepImportService();
+        ImportedFileData data = svc.Import(path);
+        System.Console.WriteLine($"=== {System.IO.Path.GetFileName(path)} ===");
+        Print(data.Root, 0);
+        return 0;
+    }
+
+    private static void Print(ImportedNode n, int depth)
+    {
+        var b = n.Bounds;
+        string box = b.IsEmpty
+            ? "(empty)"
+            : $"X[{b.X,8:F1}..{b.X + b.SizeX,8:F1}] Y[{b.Y,8:F1}..{b.Y + b.SizeY,8:F1}] Z[{b.Z,8:F1}..{b.Z + b.SizeZ,8:F1}]  " +
+              $"size {b.SizeX,7:F1} x {b.SizeY,7:F1} x {b.SizeZ,7:F1}";
+        System.Console.WriteLine($"{new string(' ', depth * 2)}{n.Name,-28} {box}");
+        foreach (ImportedNode c in n.Children) Print(c, depth + 1);
+    }
+}

tools/SmokeTest/Program.cs

@@ -32,6 +32,9 @@ if (args[0] == "--interference-test")
 if (args[0] == "--align-test")
     return SmokeTest.AlignTest.Run() == 0 ? 0 : 2;
 
+if (args[0] == "--bbox")
+    return SmokeTest.BBoxDump.Run(args[1]);
+
 var service = new StepImportService { Progress = s => Console.WriteLine($"  [階段] {s}") };
 int failed = 0;