gate-optional-effect-at-the-shared-primitive

Put a user's on/off setting for an optional decorative surface treatment inside the one shared primitive that draws it — published from the theme as a CompositionLocal defaulting to on — instead of asking every call site to check the flag. Covers why the off path must keep the shape and the hit target and change only the paint, why the default is true rather than false, and the difference between a call site that…