Implement the reviewed UI icon system and durable review workflow (#853)

Thanks @Nathan-Dane.

README.md

@@ -206,9 +206,13 @@ cargo run --release -p solidcraft-cli -- snapshot examples/bracket.json --out br
 cargo run --release -p solidcraft-cli -- render examples/render-studio.json --out product.png
 cargo run --release -p solidcraft -- part.step          # open a STEP file as a new design
 cargo run --release -p solidcraft-cli -- eval part.step # measure an imported STEP file
-cargo xtask ci                                          # fmt, clippy, tests, assets, layers, wasm
+cargo xtask ci                                          # fmt, clippy, tests, icons, assets, layers, wasm
 ```
 
+UI artwork changes follow the [icon specification](assets/icons/ui/STYLE_GUIDE.md).
+The [included review tool](assets/icons/ui/README.md) discovers new icons and keeps
+unaccepted artwork Open; run `cargo xtask icons` to check the collection.
+
 Windows builds: `cargo xwin build --release --target x86_64-pc-windows-msvc -p solidcraft`.
 
 On Windows and Linux, SolidCraft detects a connected 3Dconnexion SpaceNavigator or SpaceMouse through its