render_fall now drives the per-round drop with ease_in_cubic (P15.1 accel-from-
rest) instead of ease_out_cubic, so falling gems start slow and accelerate into
place like gravity rather than decelerating. f(1)=1 is pinned, so every gem still
lands exactly on its destination cell and move.final is untouched. FALL_ANIM_DUR
(0.22s) is unchanged, so the cascade-cue timing snapshots don't churn.
Golden goldens/p17_fall.png: M3TE_FX=11 (depth-5 cascade, seed 1337) pinned at
M3TE_ANIM_TIME=1.51 (round 3 fall window [1.38,1.60)) — gems caught bunched-high
mid-fall, ~20% down at ~59% of the segment, vs the old curve's ~93%.