Centre the disk image's icons on the new background's arrow

.claude/rules/rleapp-build-and-release.md

@@ -108,6 +108,10 @@ installs only on ARM64. macOS (Apple silicon and Intel): `.app` and `.dmg`. The
 laid out by dmgbuild from `packaging/dmg_settings.py`: the app and an Applications link
 either side of the arrow on `packaging/dmg_background.png`. The settings place the icons
 for that 960x540 image, so a new background keeps its size and its arrow where it is.
+The background of 2026-10-01 moved the arrow 44 points right (88 pixels on the @2x image),
+to a centre at x=479, and the icons with it, from (260, 290) and (610, 290) to (304, 290)
+and (654, 290). A test finds the arrow on the background and requires the two icons to
+straddle it, so a background that moves it again fails there rather than on a Mac.
 `dmg_background@2x.png` beside it, at exactly 1920x1080, is what a Retina screen shows:
 dmgbuild finds it by name and joins the two into one TIFF with `tiffutil
 -cathidpicheck`, which refuses a pair that is not exactly 1x and 2x. Without it the

admin/test/scripts/test_packaging_build.py

@@ -409,6 +409,23 @@ def test_the_window_is_as_wide_as_its_background(self):
         self.assertEqual(win_w, width)
         self.assertGreaterEqual(win_h, height)
 
+    def test_the_icons_sit_either_side_of_the_arrow(self):
+        """A new background that moves the arrow, as the one of 2026-10-01 did by 44 points,
+        leaves the icons off centre unless the settings move with it. The arrow is the only
+        dark mark in its band."""
+        from PIL import Image  # pylint: disable=import-outside-toplevel
+        with Image.open(PACKAGING / 'dmg_background.png') as img:
+            rgb = img.convert('RGB')
+            dark = [x for y in range(250, 320) for x in range(rgb.width)
+                    if sum(rgb.getpixel((x, y))) < 600]
+        arrow_centre = (min(dark) + max(dark)) / 2
+        locations = _dmg_settings()['icon_locations']
+        (app_x, app_y), (apps_x, apps_y) = locations['RLEAPP.app'], locations['Applications']
+        self.assertEqual(app_y, apps_y)
+        self.assertLess(app_x, min(dark))
+        self.assertGreater(apps_x, max(dark))
+        self.assertLessEqual(abs((app_x + apps_x) / 2 - arrow_centre), 2)
+
     def test_the_retina_background_is_exactly_twice_the_size(self):
         """dmgbuild joins dmg_background@2x.png to the background with tiffutil
         -cathidpicheck, which refuses a pair that is not exactly 1x and 2x, and the disk

packaging/dmg_settings.py

@@ -31,4 +31,7 @@
 icon_size = 144
 text_size = 14
 label_pos = "bottom"
-icon_locations = {appname: (260, 290), "Applications": (610, 290)}
+# Either side of the arrow, whose centre is at x=479 on the background. The arrow moved 44
+# points right with the background of 2026-10-01, and the icons with it, from (260, 290)
+# and (610, 290).
+icon_locations = {appname: (304, 290), "Applications": (654, 290)}