docs: add v0.7.1 release notes

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XgHf5MW39U3Xo8y2uA229K

docs/release-notes/v0.7.1.md

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+# v0.7.1 — Large STL files open in a blink
+
+## ✨ New features
+
+**Large STL files open in a blink**
+* Opening big STL meshes (tens of MB, millions of triangles) is now near-instant instead of a multi-minute wait. A 75 MB / 1.5-million-triangle STL that used to take ages now shows in **under a second**.
+* STL is now parsed directly instead of going through the CAD kernel's importer, which used to build a full B-rep face per triangle (100k triangles: **~41 s → ~11 ms**). Both binary and ASCII STL are detected and read automatically.
+
+**Customizable menu bar & quick toolbar**
+* A full menu bar groups every feature into categorized dropdowns, and a quick toolbar lets you tick just the buttons you use most — your selection is remembered between sessions.
+
+**About dialog with a changelog**
+* Help → About now shows a per-version, plain-language changelog so you can see what each release brought.
+
+## 🐛 Bug fixes
+
+**STL geometry no longer leaks into STEP export**
+* STL is pure mesh (no B-rep), so STL parts are now correctly excluded from STEP export — earlier versions wrongly mixed STL shells into the exported STEP. Rigid transforms and measurement on STL parts are unaffected.
+
+## 📦 Downloads
+
+| Build | Needs .NET Runtime? | Notes |
+|---|---|---|
+| **STPViewer_v0.7.1** (Standard) | ✅ Requires .NET 8 Desktop Runtime | Smaller download |
+| **STPViewer_v0.7.1_portable** (Portable) | ❌ Runtime included | Unzip and run anywhere, no admin |
+
+Run `STPViewer v0.7.1.exe`.
+
+## 🔗 Links
+
+* [Architecture](https://github.com/ETWen/STPViewer/blob/main/ARCHITECTURE.md)
+* [README](https://github.com/ETWen/STPViewer/blob/main/README.md)
+
+**Full changelog:** [v0.6.0...v0.7.1](https://github.com/ETWen/STPViewer/compare/v0.6.0...v0.7.1)