App icon: a fringe cube with flow streamlines replaces the meshed bracket

The mark is now a hex-meshed cube in isometric view whose faces carry a
banded colour fringe (blue to red, hottest at the front corner), with three
tapered streamlines wrapping around it. The small variant keeps the fringe
cube and one bolder streamline. Icon Composer gets four groups (floor, flow
behind, cube with mesh, flow in front), its maximum. The title-bar mark is
redrawn as the fringe cube.

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

assets/app-icon/README.md

@@ -1,10 +1,11 @@
 # FieldCraft application icon
 
-FieldCraft's mark is an L-shaped bracket with a bolt hole, drawn in isometric view and covered
-by a structured hexahedral mesh with an O-grid around the hole. Its faces are separated by
+FieldCraft's mark is a cube in isometric view, covered by a structured hexahedral mesh, whose
+faces carry a banded colour fringe (blue to red, hottest at the front corner) as a stress or
+temperature result would, with three tapered flow streamlines wrapping around it. Its faces are separated by
 transparent gaps rather than painted edge lines, so the backdrop shows through them in every
 appearance. It is original artwork, attributed in `ATTRIBUTION.md`, and shares the Craft
-family's layout (rounded-square backdrop, isometric floor grid, lit top faces) in its own teal.
+family's layout (rounded-square backdrop, isometric floor grid, lit top faces).
 
 Everything here is generated by `cargo xtask app-icon` (`xtask/src/app_icon.rs`), which computes
 the geometry and draws it with VectorCraft's command-line tool (`VECTORCRAFT_CLI`, else
@@ -14,10 +15,10 @@ hand. It runs on macOS (sips, iconutil, `xcrun actool`) with ImageMagick (`magic
 | File | What it is |
 |---|---|
 | `icon.vectorcraft`, `icon.svg`, `icon.png` | The full icon on its backdrop, 1024 × 1024 |
-| `icon-small.vectorcraft`, `icon-small.svg`, `icon-small.png` | Small-size artwork used at 16–32 px: no mesh or floor grid, faces edge to edge |
+| `icon-small.vectorcraft`, `icon-small.svg`, `icon-small.png` | Small-size artwork used at 16–32 px: the fringe cube and one bolder streamline, no mesh or floor grid, faces edge to edge |
 | `icon.iconset/`, `icon.icns` | macOS iconset (sips) and its iconutil compilation |
 | `icon.ico` | Windows icon, 16–256 px |
-| `FieldCraft.icon/` | Icon Composer document: floor grid (Default and Dark), part and mesh layers, each scaled to 90% |
+| `FieldCraft.icon/` | Icon Composer document: floor grid (Default and Dark), flow behind, cube (faces and mesh) and flow in front layers, each scaled to 90% |
 | `layers/*.vectorcraft` | Editable sources of the Icon Composer layers |
 | `Assets.car` | `actool` compilation of `FieldCraft.icon` for macOS 26 and later (needs Xcode 26 or later; the task skips it with a note otherwise) |