GlassSheetDetent enum

The resting heights a sheet may stop at, chosen via GlassModalSheet.detents. Mirrors UIKit's sheet detents:

• medium — the content-height, translucent-glass stop (Apple Pay / Sign in with Apple). Maps to GlassSheetState.half internally. • large — the screen-height, opaque stop (Maps / Music). Maps to GlassSheetState.full.

A sheet declares any non-empty subset: {medium} is half-only glass, {large} is full-only opaque, {medium, large} is the default two-stop sheet, and adding small puts a peek floor underneath.

small is the maps-style resting floor — the state a persistent sheet returns to instead of dismissing. Its styling lives in the top-level peek* params (GlassModalSheet.peekSettings, peekWidth, …) rather than on the detent itself, deliberately: a Set whose members carried per-instance payload would need equality that IGNORES that payload, or detents.contains(small) stops working and two smalls with different settings could sit in one set at once. Keeping the detent a plain enum keeps set membership meaning exactly one thing.