Bump to v0.2.1 (maintenance): CI now verifies the NSIS installer contents via 7z

- Add a 'Verify Windows installer contents' step (7z l shows Step Line.exe +
  uninstall.exe) so the TODO verification items are proven by CI output
- Rebuild 0.2.1 binaries (embedded PCK) and AppImage locally; smoke clean;
  57/57 tests

.gitea/workflows/build.yml

@@ -79,13 +79,19 @@ jobs:
 
       - name: Package Windows installer (NSIS)
         run: |
-          sudo apt-get update && sudo apt-get install -y nsis
+          sudo apt-get update && sudo apt-get install -y nsis p7zip-full
           VERSION="$(sed -n 's/.*config\/version="\([^"]*\)".*/\1/p' project.godot)"
           makensis /DVERSION="$VERSION" \
             /DARTIFACT="build/step-line-windows-$VERSION.exe" \
             /DICON=packaging/windows/step-line.ico \
             packaging/windows/step-line.nsi
 
+      - name: Verify Windows installer contents
+        run: |
+          VERSION="$(sed -n 's/.*config\/version="\([^"]*\)".*/\1/p' project.godot)"
+          ls -la build/step-line-setup-$VERSION.exe
+          7z l build/step-line-setup-$VERSION.exe | grep -E "Step Line.exe|uninstall.exe"
+
       - name: Package Linux AppImage
         run: |
           bash packaging/linux/build_appimage.sh