Adopt pure-black surface elevation behavior in MMDS components

[georgewrmarshall]

Description

Audit elevated MMDS components and internalize pure-black-specific surface logic where token changes alone are insufficient.

Acceptance Criteria

• BottomSheetDialog, Modal, and other elevated surfaces are audited
• Components render appropriate elevated backgrounds in pure-black mode
• Consumers do not need per-instance pure-black conditionals
• Existing dark mode appearance remains unchanged otherwise

Constraint

• isPureBlack=false uses the existing grey dark theme
• isPureBlack=true uses pureBlackDarkTheme

References

• Jira epic: TMCU-622