feat(theme): add semantic container color prop to Surface

[hristototov]

Motivation

Surface derives its background from the numeric elevation prop, so a component cannot express "this container is surfaceContainerLow, and elevation only controls the shadow" without passing a raw color. This adds a first-class semantic API for it:

• New container prop on Surface accepting a surface-family color role (surface, surfaceDim, surfaceBright, surfaceVariant, surfaceContainerLowest-Highest, inverseSurface), resolved from the theme. Precedence: backgroundColor > container > elevation-derived color.
• When container is set, elevation only affects the shadow. Animated elevation no longer interpolates the background.
• Default behaviour is unchanged; no migration needed for existing consumers.
• Menu (surfaceContainer, per MD3 spec) and Banner (surfaceContainerLow) migrated onto it. Pixel-identical at their default elevations since elevation.level1-5 map to the same tones. Consumers using non-default elevations now get only a shadow change (noted in the migration guide).
• New public SurfaceRole type.

Related issue

Fixes #5119.

Screenshots / Videos

light-mode.mov

dark-mode.mov

Test plan

• yarn typecheck, yarn lint, yarn test — all green (55 suites, 693 tests; no snapshot diffs, confirming the Menu/Banner migrations are visually unchanged).
• New Surface tests