docs: Add v0.5.0 release notes

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/release-notes/v0.5.0.md

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+# v0.5.0 — Hole-pitch measuring, 3-point section planes and STEP export
+
+## ✨ New features
+
+**Measure hole-to-hole pitch in two clicks — snap to circle centers**
+* Point / Distance / Align picks now **snap to the center of a circular edge** when you click near
+  a hole or a round boss — measure connector pin pitch by simply clicking two hole rims.
+* Vertex snapping still works; whichever feature is closest to your click wins.
+
+**Keyboard shortcuts and Esc**
+* Measure modes now have hotkeys: **P** point, **D** distance, **E** edge, **F** face, **C** circle,
+  **A** angle, **M** face-to-face. Press the same key again to go back to browsing.
+* **Esc** cancels an in-progress multi-step measurement first (keeping the mode), then exits the
+  mode, then closes the manipulator — no more re-clicking toolbar buttons after a mis-pick.
+
+**Standard views and orthographic projection**
+* One-click **Iso / Front / Top / Right** view buttons (Z-up, mechanical convention).
+* New **orthographic ⇄ perspective** toggle — check dimensions without perspective distortion.
+  The 3-axis manipulator stays perfectly aligned in both projections.
+
+**Assembly tree: search, isolate, volume**
+* A **search box** above the tree filters nodes by name (ancestors and children of a match stay
+  visible, branches auto-expand) — find one part in a hundred instantly.
+* Right-click any node: **Isolate** (show only this node), **Invert visibility**, **Show all**.
+* Right-click **Volume / centroid**: closed solids get mesh-based volume, a centroid marker in the
+  view, and overall L×W×H. Node tooltips now show each part's bounding size too.
+
+**Cut along any plane — 3-point section + numeric position**
+* The section axis picker gains a **"3-pt"** option: click three points on the model to define an
+  arbitrary cutting plane, then sweep it with the position slider as usual.
+* The position value is now also an **editable number field** (0–100 %).
+
+**Interference check across 3+ files**
+* The check now accepts **two or more** visible files and tests **every pair**, reporting each
+  pair's result (intersection lines or minimum gap) plus a summary of how many pairs collide.
+
+**Hand off your alignment — export the scene as STEP**
+* New **📤 STEP** button writes all visible parts **at their current aligned positions** into a
+  brand-new STEP file — mate connectors in STPViewer, then hand the assembly to any CAD.
+  Original files are never modified.
+
+**Quality of life**
+* Window size/position, mm ⇄ inch choice and the **last 10 opened files** (▾ next to Import) are
+  remembered between sessions.
+* The tree and measurement panels are now **resizable** with drag splitters.
+
+## 📦 Downloads
+
+Two builds are produced:
+
+| Build | Needs .NET 8 Desktop Runtime? | Notes |
+|-------|-------------------------------|-------|
+| **Standard** (`STPViewer_v0.5.0`) | ✅ Yes | Smaller; for machines that already have the runtime |
+| **Portable** (`STPViewer_v0.5.0_portable`) | ❌ No | Runtime bundled — unzip and run, no install / admin needed |
+
+Run `STPViewer v0.5.0.exe`.
+
+**Full changelog:** [v0.4.0...v0.5.0](https://github.com/ETWen/STPViewer/compare/v0.4.0...v0.5.0)