Function run_fast_path_cpu 

pub fn run_fast_path_cpu(
    current: &[[f32; 3]],
    history: &[[f32; 3]],
    residual_history_in: &[f32],
    drift_history_in: &[f32],
    width: usize,
    height: usize,
    lambda: f32,
    k: f32,
    local_aggregation: bool,
) -> FastPathCpuOutput

CPU reference implementation of the minimal inline fast-path proxy.

Inputs are flat slices of per-pixel [R, G, B] triples. All output Vecs are pre-allocated before the inner loop; no allocations occur inside the per-pixel computation.

This function is deterministic: identical inputs produce identical outputs.