macOS DMG: branded window background and icon layout

The DMG opened as a plain Finder window. It now shows the app icon's figure on
the EffectCraft colour, with EffectCraft.app and the Applications link side by
side.

- packaging/macos/dmg/: background.svg (source; text outlined, the figure an
  <image> of assets/app-icon) and dmg-layout.DS_Store (Finder view settings:
  window, icon size 128, positions, background), written by generate.py on any
  OS, byte-for-byte reproducible, with nothing from the machine that ran it.
- package.sh renders the SVG with resvg (1x + 2x) into a HiDPI TIFF at package
  time, so no bitmap is committed; release.yml installs resvg 0.48.1 (pinned,
  --locked) on the macOS runner. The image is still built with hdiutil
  makehybrid.
- The volume name drops the version: the layout finds the background through
  an alias that includes the volume name. The DMG file name keeps the version.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.github/workflows/release.yml

@@ -88,6 +88,9 @@ jobs:
           APPLE_CERTIFICATE_PASSWORD: ${{ secrets.APPLE_CERTIFICATE_PASSWORD }}
           KEYCHAIN_PASSWORD: ${{ secrets.KEYCHAIN_PASSWORD }}
         run: packaging/macos/import-cert.sh
+      # Renders the DMG background from packaging/macos/dmg/background.svg (package.sh).
+      - name: Install resvg
+        run: cargo install resvg --locked --version 0.48.1
       - name: Build, sign, notarize
         env:
           APPLE_ID: ${{ secrets.APPLE_ID }}