Fix elevation tint hiding consumer backgrounds on Paper components

[zoharma]

Summary

Fixes #318: sx background colours on Card, Accordion, and other elevated Paper components had no visible effect. DiamondDS paints elevation as an opaque background-image, which always paints above background-color, so no consumer colour could show through.

Fix

• MuiPaper: paint tonal elevation as backgroundColor instead of backgroundImage, guarded to elevation > 0 so Alert(elevation={0}) keeps its own background
• MuiAutocomplete: same change, routed through the same surface.elevated() helper as MuiPaper instead of a duplicated token string
• surface.elevated() now rounds before clamping, so a fractional elevation degrades to the nearest valid step instead of silently dropping the tint
• Token values unchanged, only how the tint is painted

Moving the tint to background-color gives exact, computable surface colours, similar to M3's elevation direction.

Examples

Before (#318 bug: background-image painted over the colour)

After (fix: background-color, consumer colour wins)
Following Material 3 style.

For comparison (translucent tint over the same colour)
Mirrors MUI / Material 2 dark-mode overlay direction, in light mode. Elevation keeps a visible effect even with a custom colour set, and provides a "dirty" colour.

Testing

• Regression tests for the new override shape
• New render-based tests (jsdom can't catch paint-order bugs, but can catch cascade-order ones): a consumer sx background wins over the tint, Alert keeps its own background at elevation={0}, Autocomplete's listbox gets its own elevation tier over MuiPaper's default, and a slotProps.paper.sx background (NavMenu) still wins
• Verified in a real browser for the paint-order bug itself: red sx background was invisible pre-fix, renders correctly post-fix in both light and dark

Storybook

• Added a CustomBackground story to Paper (elevation + a custom colour). No existing Paper/Card/Accordion story exercised this
• Bumped a couple of stories off elevation={1}/2 where the tonal tint was too subtle to actually see