perf: speed up distinct index collection in low-memory pipeline

[hjanuschka]

This removes Option staging in get_distinct_indices and builds the result directly in position order. It reduces overhead in a hot helper used during low-memory pipeline row setup without changing behavior. Unsafe is used only to reborrow previously collected mutable pointers, and is safe because indices are distinct and sorted, so aliasing cannot occur.

[github-actions]

Benchmark @ e86a453

MULTI-FILE BENCHMARK RESULTS (4 files)
  CPU architecture: x86_64
  WARNING: System appears noisy: high system load (2.13). Results may be unreliable.
Statistics:
  Confidence:               99.0%
  Max relative error:        3.0%

Comparing: e883140e (Base) vs 0915daeb (PR)

File	Base (MP/s)	PR (MP/s)	Δ%
bike.jxl	24.654	24.585	-0.28% ±2.6%
green_queen_modular_e3.jxl	7.923	7.917	-0.07% ±0.6%
green_queen_vardct_e3.jxl	24.099	23.776	-1.34% ±1.4%
sunset_logo.jxl	2.792	2.772	-0.73% ±1.5%