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 SoundCraft colour, with SoundCraft.app and the Applications link side by side.
Same approach as PhotoCraft's and VectorCraft's (storytold/photocraft#1201,
storytold/vectorcraft#493).

- packaging/macos/dmg/: background.svg (source; text outlined, no paper grain,
  the figure an <image> of assets/app-icon), background.tiff (1x + 2x HiDPI,
  16-colour palette, Deflate, sRGB; ~28 KB)
  and dmg-layout.DS_Store (Finder view settings: window, icon size 128,
  positions, background).
- generate.py writes the TIFF and the layout on any OS, byte-for-byte
  reproducible, with nothing from the machine that ran it (no paths, users,
  disk names or volume UUIDs).
- package.sh copies them into the staging folder; the image is still built with
  hdiutil makehybrid. Nothing extra runs in the signing job.
- 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 <noreply@anthropic.com>

docs/releasing.md

@@ -94,6 +94,9 @@ plugins), and WebAudio in the browser.
 - **Notarization:** the app is zipped and sent with `xcrun notarytool submit --wait`, then the
   ticket is stapled to the app. The app goes on a DMG (`hdiutil`, with an `Applications` link).
   The DMG is signed, notarized and stapled too.
+  Its Finder window (background, icon size and positions) comes from
+  [`packaging/macos/dmg/`](../packaging/macos/dmg/README.md), and its volume is named `SoundCraft`
+  without the version, which the window's background needs; the DMG file name keeps the version.
 - **CLI:** the universal `soundcraft-cli` is signed with the hardened runtime, zipped and
   notarized.