feat(assets): the real Atelier artwork, replacing the amber swatches

Both masters already existed in OfficerIcons — the painter duck with the
beret and palette — so the plugin was showing a flat #f59e0b square for want
of a copy step, not for want of art.

The icon master is 1312x1199, not square, so it is trimmed to the artwork
before it is fitted to 256: resizing that canvas directly would have drawn the
duck at about four fifths the size everything else in the dock uses. Splash is
a straight 1254 -> 1024, and PLACEHOLDER.md goes with them.

assets/PLACEHOLDER.md

@@ -1,9 +0,0 @@
-# [placeholder] artwork
-
-`icon.png` (256×256) and `splash.png` (1024×1024) are flat `#f59e0b` swatches — the manifest's accent
-colour, nothing more. No real Atelier artwork existed to carry over from the standalone service, and
-drawing one is outside an unattended run's scope.
-
-Replace both with real artwork before this plugin is published to the Marketplace; until then the dock
-tile and any mobile launch screen will show a plain amber square. Grep for `[placeholder]` across this
-plugin to find every other invented value.