rdoq: skip uncoded cost setup for trailing zero groups

[xiujunwang-dev]

When the final coefficient group precedes trailing zero groups, rdoQuant currently initializes uncoded distortion for every coefficient. The trailing groups add the same cost to every candidate, so this omits common work while preserving RDOQ decisions.

Validation:

• Quant::rdoQuant function-level timing on arm64, five paired runs (ns/call):
  • HD input, 16x16: 418.5 -> 406.9 (+2.9%), faster in 5/5 runs.
  • HD input, 32x32: 1630.1 -> 1484.9 (+9.8%), faster in 5/5 runs.
  • Portrait input, 16x16: 634.5 -> 618.4 (+2.6%), faster in 3/5 runs.
  • Portrait input, 32x32: 2863.8 -> 2666.5 (+7.4%), faster in 5/5 runs.
  • 4x4 and 8x8 show no stable improvement.
• 36 paired RDOQ output checks passed across bit depths and modes.