revert(producer): drop renderStretch fit_to_scene retime (#2676)

[xuanruli]

Reverts #2676 (renderStretch).

Why

renderStretch tried to fix the fit_to_scene stutter (a composition shorter than its scene is frame-held by the compositor) by re-timing the GSAP timeline at render: emit target-length output and scale the per-frame seek across [0, intrinsic]. Local render tests show this cannot work — seeking a 1s-authored timeline more finely does not create new motion. The extra output frames are near-duplicates of their neighbours (sub-pixel deltas collapse on capture), so a 1s composition still yields ~30 distinct frames regardless of renderStretch. Measured: a 1s composition renders 30 unique frames at 30fps and stays ~30 unique under renderStretch; only 118 unique at a native 120fps render.

What replaces it

The correct fix needs no producer change: the producer already honours the fps override, and rendering a short composition at a scene-scaled fps (ceil(output_fps * scene / intrinsic)) produces genuinely distinct frames that fill the stretched clip. That lives entirely in experiment-framework (#42766) — it computes the fps and passes it through the existing fps field. No renderStretch plumbing required.

Ref: VA-1859.