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.

tests/expected/gem_pose.stdout

@@ -7,7 +7,7 @@ select_start_rest true select_end_rest true select_mid_pops true
 == land squash envelope ==
 land_start_rest true land_end_rest true land_mid_wobbles true
 == clear pop envelope ==
-clear_start_full true clear_end_gone true clear_overshoots true
+clear_start_full true clear_end_gone true clear_dips true clear_overshoots true clear_collapses true
 == gem motion land bookkeeping ==
 motion_init true motion_no_land true motion_fresh_land true
 == gem motion restart resets landings ==