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/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;