P17.3: organic fall — per-round landing squash-&-settle (sx, iOS sim)

Give each landing gem a wide-and-short squash-&-settle bounce as it touches
its destination, applied WITHIN the fall so EVERY cascade round bounces
(staggered per column), not only the final whole-move settle.

One envelope, one bounce: land_squash is now LAND_SQUASH_A * squash_envelope
(P15.1) over its normalized window, so the per-round fall bounce and the
settle bounce are the exact same shape. render_fall/render_clear age a
per-column bounce from each column's touch-down instant (fall_landing_frac *
FALL_ANIM_DUR) via the shared rest_squash + delivering_round helpers, so a gem
still in the air draws unsquashed and only a landed gem flattens; the squash
carries across the fall->clear seam.

Double-bounce reconciliation (approach a): drive the bounce from the per-round
fall and DROP the old whole-move "stamp at age 0" settle. The settle stamp is
now BACK-DATED per column (clock - (total - round_land_time)) so render_gems
resumes land_squash exactly where render_fall left off at the render_anim ->
render_gems seam — one continuous bounce, no double-pop.

Amplitude tuned 0.13 -> 0.18 (~13% peak) so the bounce reads while staying
tasteful; durations unchanged, so the cascade-cue snapshots don't churn.
M3TE_ANIM_TIME=0 still reproduces goldens/p6_idle_t0.png (a resting board
carries no landing stamp). New goldens/p17_land.png pins a staggered landing
mid-pour (M3TE_FX=11 ANIM_TIME=1.94). tests/easing.sx gains a landing-instant
section pinning fall_landing_frac / round_land_time; tests/gem_pose.sx stays
green (land_squash values are identical).

gem_anim.sx

@@ -12,6 +12,7 @@
 #import "modules/std.sx";
 #import "modules/math";
 #import "board.sx";
+#import "board_anim.sx";
 
 // A gem's draw transform about its cell centre. scale_x/scale_y scale the sprite
 // (1.0 == the normal cell-fill size) and dx/dy nudge it in CELL units. The resting
@@ -63,14 +64,15 @@ select_pop_scale :: (ts: f32) -> f32 {
 // --- Landing squash-bounce ---------------------------------------------------
 // A damped wobble on settle: the gem flattens wide-and-short on impact, then a
 // couple of decaying overshoots. 0 at tl==0 and again past the window (rest).
+// The shape IS P15.1's `squash_envelope` (board_anim) over the normalized window
+// tl/LAND_DUR, scaled to a tasteful amplitude — so the per-round fall bounce and
+// this settle bounce are the EXACT same envelope (P17.3 drives both through here).
 LAND_DUR      :f32: 0.42;
-LAND_SQUASH_A :f32: 0.13;
-LAND_OSC      :f32: 1.5;    // oscillations across the window
+LAND_SQUASH_A :f32: 0.18;   // peak ~13% wide-and-short — reads on landing, still tasteful
 
 land_squash :: (tl: f32) -> f32 {
     if tl <= 0.0 or tl >= LAND_DUR { return 0.0; }
-    decay := 1.0 - tl / LAND_DUR;
-    LAND_SQUASH_A * sin(TAU * LAND_OSC * tl / LAND_DUR) * decay * decay
+    LAND_SQUASH_A * squash_envelope(tl / LAND_DUR)
 }
 
 // --- Clear pop ---------------------------------------------------------------
@@ -117,7 +119,15 @@ GemMotion :: struct {
     }
 
     stamp_land :: (self: *GemMotion, i: s64) {
-        self.land_at[i] = self.clock;
+        self.stamp_land_at(i, self.clock);
+    }
+
+    // Record cell `i`'s landing at an explicit clock value, so the settle hand-off
+    // can BACK-DATE the stamp to when the gem actually touched down mid-fall (each
+    // column lands at a staggered instant): land_squash then resumes the per-round
+    // bounce exactly where render_fall left it, with no double-pop at the seam.
+    stamp_land_at :: (self: *GemMotion, i: s64, at: f32) {
+        self.land_at[i] = at;
     }
 
     land_local :: (self: *GemMotion, i: s64) -> f32 {