Use ka/Ma unit suffixes for geological-scale year grouping headings
Display human-readable units in YearGrouping headings for large time spans:
- Ma (mega-annum) when time_span >= 1M years,
- ka (kilo-annum) when time_span >= 10k years.
Smaller spans (decades, centuries, millennia)
keep the existing 's' suffix for readability.
Examples: -80000000s → -80 Ma, -10000s → -10 ka, 1000s stays 1000s.
Bug: T236590