perf: use StackVec in render channel plumbing

[hjanuschka]

This introduces a fixed-capacity StackVec and uses it in render channel/row hot paths instead of SmallVec. It removes stack-vs-heap branch overhead in frequently executed indexing and row collection paths while keeping behavior unchanged. Unsafe in StackVec is only for MaybeUninit element access and is safe because reads are restricted to initialized elements in 0..len and writes respect capacity invariants.

[github-actions]

Benchmark @ 38d6610

MULTI-FILE BENCHMARK RESULTS (4 files)
  CPU architecture: x86_64
  WARNING: System appears noisy: high system load (2.37). Results may be unreliable.
Statistics:
  Confidence:               99.0%
  Max relative error:        3.0%

Comparing: e883140e (Base) vs c31adff2 (PR)

File	Base (MP/s)	PR (MP/s)	Δ%
bike.jxl	24.478	24.922	+1.82% ±2.9%
green_queen_modular_e3.jxl	7.910	7.805	-1.33% ±1.2%
green_queen_vardct_e3.jxl	24.044	24.188	+0.60% ±0.7%
sunset_logo.jxl	2.784	2.767	-0.63% ±0.4%