Function formatDisplayNumber

Format a number into structured display parts using tier-based rules.

Tiers:

1. num < 0.0001 → subscript notation: 0.0ₙXXXX (4 sig digits)
2. 0.0001 ≤ num < 1 → 4 significant digits
3. 1 ≤ num < 100 → up to 4 decimal places
4. 100 ≤ num < 10,000 → up to 2 decimal places + grouping
5. 10,000 ≤ num < 1,000,000 → integer + grouping
6. ≥ 1,000,000 → compact (M/B/T) + 2 decimal places

When fractionDigits is set, tiers are bypassed and exactly that many fraction digits are shown.