P17.1: organic fall — accelerate under gravity (ease_in_cubic)

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%.

board_anim.sx

@@ -23,8 +23,8 @@ SWAP_ANIM_DUR  :f32: 0.16;
 CLEAR_ANIM_DUR :f32: 0.14;
 FALL_ANIM_DUR  :f32: 0.22;
 
-// Easing helpers. Slide/fall decelerate into place (ease-out cubic); the clear
-// scale-out accelerates as it shrinks (ease-in quad).
+// Two base easing helpers, each with locked endpoints f(0)=0 and f(1)=1:
+// ease_out_cubic decelerates into its endpoint, ease_in_quad accelerates from rest.
 ease_out_cubic :: (t: f32) -> f32 { u := t - 1.0; u * u * u + 1.0 }
 ease_in_quad :: (t: f32) -> f32 { t * t }