The window icon

A windowed player's title bar and taskbar show an icon: a Unity project's own (ProjectSettings.asset's m_BuildTargetIcons: the largest icon for Standalone, else the default one, decoded by the importer), else game.json's icon (cargo xtask package --icon <image> copies one in), else docs/branding/tassix-icon.svg rasterized at 256 px with resvg.
Checked: a unit test of the rasterized icon; windowed runs whose window answers WM_GETICON with small and large icons; Beltz's 1254 px project icon is the one chosen.
Not yet: the executable's own icon in Explorer (a resource in the .exe).
Next: UI Toolkit's remaining pieces (TextField, sliders and scrollers, rounded corners, transitions); CompositeCollider2D.offsetDistance; the executable's icon.