P18.1: organic combine — anticipation pop on match clear (sx, iOS sim)

Sharpen clear_pop_scale from a plain pop-then-shrink into a candy pop in
three beats over its local 0..1: a tiny anticipation squash dip (~8% below
rest), a snappy overshoot to ~1.40x via P15.1's ease_out_back, then an
accelerating collapse to nothing (ease_in_quad). Endpoints stay locked
(t=0 -> 1.0 rest, t=1 -> 0.0 gone), so the seam to the model board is clean
and M3TE_ANIM_TIME=0 still reproduces the rest board; the particle burst /
score popup compose on top unchanged. render_clear and CLEAR_ANIM_DUR are
untouched, so cascade-cue timing snapshots don't churn and the model is
unchanged.

tests/gem_pose.sx now pins the new envelope (locked rest endpoints, the
anticipation dip, the overshoot, the monotonic post-peak collapse) with its
expected snapshot updated.

goldens/p18_pop.png: new mid-clear pop golden at the overshoot peak
(M3TE_FX=3 M3TE_ANIM_TIME=0.21). goldens/p6_fx_match.png refreshed for the
new pop shape (same scene at its documented 0.22). README documents P18.1.

gem_anim.sx

@@ -76,22 +76,39 @@ land_squash :: (tl: f32) -> f32 {
 }
 
 // --- Clear pop ---------------------------------------------------------------
-// The matched-gem clear: a snappy outward pop then a collapse to nothing over its
-// local 0..1, so the clear reads as a satisfying candy pop rather than a plain
-// shrink. A fast rise to a bigger overshoot makes the snap read; the soft
-// particle burst / score popup (board_fx.sx) compose on top.
-CLEAR_POP_A    :f32: 0.34;   // overshoot height above resting scale
-CLEAR_POP_RISE :f32: 0.18;   // fraction of the window spent rising to the peak
+// The matched-gem clear, shaped as a candy pop in three beats over its local
+// 0..1: a tiny anticipation squash dip (a "gather" just below rest), a snappy
+// overshoot up to the peak via P15.1's ease_out_back, then an accelerating
+// collapse to nothing. Endpoints are LOCKED — t==0 -> 1.0 (rest) and t==1 -> 0.0
+// (gone) — so the seam to the model board stays clean; the soft particle burst /
+// score popup (board_fx.sx) compose on top.
+CLEAR_DIP_T    :f32: 0.16;   // fraction of the window spent on the anticipation dip
+CLEAR_DIP_A    :f32: 0.08;   // how far the gem compresses below rest before popping
+CLEAR_POP_RISE :f32: 0.52;   // window fraction at which the overshoot peak is reached
+CLEAR_POP_A    :f32: 0.36;   // overshoot height above resting scale
 
 clear_pop_scale :: (t: f32) -> f32 {
     if t <= 0.0 { return 1.0; }
     if t >= 1.0 { return 0.0; }
-    if t < CLEAR_POP_RISE {
-        return 1.0 + CLEAR_POP_A * (t / CLEAR_POP_RISE);
+    if t < CLEAR_DIP_T {
+        // Anticipation gather: sin(PI*u) is 0 at both ends, so t==0 stays exactly
+        // at rest and the dip hands off to the rise at rest — a brief squash, not
+        // a step.
+        u := t / CLEAR_DIP_T;
+        return 1.0 - CLEAR_DIP_A * sin(PI * u);
     }
+    if t < CLEAR_POP_RISE {
+        // Snap up to the peak: ease_out_back rises from rest, shoots a touch past
+        // 1+A, then eases back to exactly 1+A at the seam (its locked f(1)=1), so
+        // the maximum is a single clean overshoot with no second reversal.
+        u := (t - CLEAR_DIP_T) / (CLEAR_POP_RISE - CLEAR_DIP_T);
+        return 1.0 + CLEAR_POP_A * ease_out_back(u);
+    }
+    // Collapse to nothing: accelerate the shrink from the peak so the gem vanishes
+    // as the burst takes over. ease_in_quad pins the seam at the peak and t==1 at 0.
     peak := 1.0 + CLEAR_POP_A;
     u := (t - CLEAR_POP_RISE) / (1.0 - CLEAR_POP_RISE);
-    peak * (1.0 - u * u)
+    peak * (1.0 - ease_in_quad(u))
 }
 
 // Live per-gem animation state, heap-allocated (like BoardAnim/BoardFx) so it