P16.1: organic legal swap — overshoot/settle (ease_out_back)

Replace the legal swap's flat ease_out_cubic decelerating slide with the
P15.1 ease_out_back overshoot curve: the two swapped gems shoot ~10% PAST
their target cells, then settle exactly onto them. Purely visual — the curve
pins f(0)=0 / f(1)=1, so t==0 is the rest pose and t==1 lands byte-on-cell;
the committed move and final board are unchanged. SWAP_ANIM_DUR (0.16 s) is
untouched, so the cascade-cue timing snapshots do not churn.

Only the legal branch of render_swap changes; the illegal ping-back is left
as-is for P16.2. Model/logic untouched (FFI is the only non-sx surface).

Golden goldens/p16_swap.png: M3TE_FX=3 (top-row swap (5,0)<->(6,0)) pinned at
M3TE_ANIM_TIME=0.10 (swap-phase t~0.625, near the overshoot peak). Measured
against calibrated cell centers: the red lands ~8% left of col-5 and the green
~12% right of col-6 (both PAST target), while every unswapped gem stays
centered; at M3TE_ANIM_TIME=0 the same gems sit dead-on their pre-swap cells.

Gate: `sx build --target ios-sim main.sx` links clean; `tools/run_tests.sh`
22/22 (anim_plan, easing, cascade_rounds, cascade_cue all green).

board_view.sx

@@ -415,7 +415,11 @@ BoardView :: struct {
 
         p : f32 = ---;
         if mv.legal {
-            p = ease_out_cubic(t);
+            // Overshoot-and-settle: the two gems shoot a touch PAST their target
+            // cells, then settle exactly onto them, instead of decelerating flatly
+            // into place. ease_out_back pins f(0)=0 and f(1)=1, so t==0 is the rest
+            // pose and t==1 lands byte-on-cell — the swap stays purely visual.
+            p = ease_out_back(t);
         } else if t < 0.5 {
             p = ease_out_cubic(t * 2.0);
         } else {