diff --git a/docs/release-notes/v0.3.1.md b/docs/release-notes/v0.3.1.md
new file mode 100644
index 0000000..494c9d0
--- /dev/null
+++ b/docs/release-notes/v0.3.1.md
@@ -0,0 +1,56 @@
+# v0.3.1 — First public release: 3D STEP viewer with measurement & assembly fit-check
+
+A lightweight Windows desktop viewer for STEP / STL / DXF — open a part, measure it,
+and check how two parts fit together, without firing up a heavyweight CAD suite.
+
+## ✨ New features
+
+**View & assembly tree**
+* Import multiple CAD files at once (STEP / STL / DXF) via the toolbar, drag-and-drop,
+  or the command line (`STPViewer.exe a.stp b.stp`).
+* STEP product structure is rebuilt into an **assembly tree** — toggle visibility, recolor
+  (cascades to children), and zoom-to any node; whole-file remove and outline-edge toggle.
+* Smooth orbit / zoom / pan even on large assemblies (hundreds of thousands of triangles).
+
+**Measurement** (pick a mode, click the model)
+* Point (snaps to nearby vertices), two-point distance + ΔX/ΔY/ΔZ, edge length
+  (line / curve / arc with radius), face area + surface type, circle center / radius /
+  diameter / circumference, angle between two faces or edges, and face-to-face minimum gap.
+* Edge length, radius and angle use **exact B-rep values**; area and gap are mesh-based.
+* Switch units **mm ⇄ inch** anytime — existing measurements and 3D labels convert live.
+
+**Assembly fit-check** (mate a male/female pair and verify)
+* **Two-point align** — click a point on the part to move, then the target point, to slide
+  the whole file into place.
+* **Three-point align** — pick 3 feature points on each file to solve a full rotate+translate
+  in one shot, for parts that aren't already oriented the same way.
+* **Axis rotate** — rotate the selected file ±90° about X / Y / Z to square it up.
+* **Drag mode** (hand cursor) — left-press a part and drag it along the screen plane.
+* **Gizmo** — XYZ arrows to move along an axis (independent of view angle, ideal for pushing
+  along a mating axis) and rings to rotate any angle. The gizmo always renders on top, so it
+  is never hidden inside a solid part.
+* **Interference check** — for two visible files, shows red intersection lines where they
+  collide, or the minimum gap when they don't (gap ≈ 0 means a clean fit).
+
+**Section & export**
+* Section view along X / Y / Z with a position slider and flip, to inspect mating internals.
+* Export measurements to CSV (opens cleanly in Excel) and save a 2× PNG screenshot of the view.
+
+## 📦 Downloads
+
+Two builds are produced:
+
+| Build | Needs .NET 8 Desktop Runtime? | Notes |
+|-------|-------------------------------|-------|
+| **Standard** (`STPViewer_v0.3.1`) | ✅ Yes | Smaller; for machines that already have the runtime |
+| **Portable** (`STPViewer_v0.3.1_portable`) | ❌ No | Runtime bundled — unzip and run, no install / admin needed |
+
+Run `STPViewer v0.3.1.exe`.
+
+## 🔗 Links
+* Overview & usage:
+  [README.md](https://github.com/ETWen/STPViewer/blob/main/README.md)
+* Architecture & design:
+  [ARCHITECTURE.md](https://github.com/ETWen/STPViewer/blob/main/ARCHITECTURE.md)
+
+**Full changelog:** first public release — [commit history](https://github.com/ETWen/STPViewer/commits/v0.3.1)
diff --git a/tools/SmokeTest/BBoxDump.cs b/tools/SmokeTest/BBoxDump.cs
new file mode 100644
index 0000000..5a21384
--- /dev/null
+++ b/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);
+    }
+}
diff --git a/tools/SmokeTest/Program.cs b/tools/SmokeTest/Program.cs
index f26a8d3..7960ce9 100644
--- a/tools/SmokeTest/Program.cs
+++ b/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;