Style the DMG: dmgbuild layout, background, large icons

Layout is written by dmgbuild (Config/dmg/settings.py) instead of Finder
AppleScript, which proved unreliable. Background is generated by
scripts/make-dmg-background.swift.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Config/dmg/settings.py

@@ -0,0 +1,23 @@
+# dmgbuild settings: writes the Finder window layout directly (no Finder/AppleScript needed).
+# Keep coordinates in sync with scripts/make-dmg-background.swift.
+app = defines['app']  # noqa: F821  (injected by dmgbuild -D)
+
+format = 'UDZO'
+filesystem = 'HFS+'
+files = [app, 'Config/dmg/Read Me First.txt']
+symlinks = {'Applications': '/Applications'}
+background = 'Config/dmg/background.png'
+
+window_rect = ((200, 120), (726, 720))
+default_view = 'icon-view'
+show_toolbar = False
+show_status_bar = False
+show_sidebar = False
+icon_size = 128
+text_size = 13
+arrange_by = None
+icon_locations = {
+    'SecondDisplay.app': (198, 195),
+    'Applications': (529, 195),
+    'Read Me First.txt': (363, 435),
+}