P7.2 fix: reset per-gem landing state on restart

The restart button (BoardView.do_restart) reseeded the model and dropped
selection/drag/anim/FX, but left GemMotion.land_at carrying the prior move's
landing stamps. A restart fired right after a terminal cascade therefore
replayed that move's squash-bounce on the freshly seeded board instead of
showing a clean resting pose.

Factor the landing reset into GemMotion.reset_landings (init now delegates to
it) and call it from do_restart, so a restart returns every cell to its
resting idle pose. The idle clock keeps running, so the always-on idle simply
resumes from rest.

Regression: tests/gem_pose.sx section 7 stamps a cell mid-squash, asserts it
is squashing, then asserts reset_landings returns every cell to rest while
leaving the clock untouched. Fails on the pre-fix (no-op reset) behaviour,
passes after. Gate green: ios-sim build + 18/18 logic tests.

gem_anim.sx

@@ -103,6 +103,14 @@ GemMotion :: struct {
     init :: (self: *GemMotion) {
         self.clock = 0.0;
         self.pinned = false;
+        self.reset_landings();
+    }
+
+    // Drop every landing stamp back to the never-landed sentinel so no cell
+    // carries a squash-bounce. `restart` calls this so a reseeded board starts at
+    // its resting pose instead of replaying the prior move's landing wobble; the
+    // idle clock keeps running, so the always-on idle simply resumes from rest.
+    reset_landings :: (self: *GemMotion) {
         for 0..BOARD_CELLS: (i) { self.land_at[i] = -1000.0; }
     }