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.

board_view.sx

@@ -487,14 +487,17 @@ BoardView :: struct {
 
     // Restart action behind the banner's button: reseed the SAME starting level
     // through the model (board.restart) and drop every transient view layer
-    // (selection, in-flight drag, move animation, FX) so the board returns to a
-    // clean in_progress state.
+    // (selection, in-flight drag, move animation, FX, and the per-gem landing
+    // bounce) so the board returns to a clean, resting in_progress state. Without
+    // the motion reset a restart fired right after a terminal cascade would carry
+    // that move's landing squash onto the freshly seeded board.
     do_restart :: (self: *BoardView) {
         self.board.restart(self.seed);
         self.sel.clear();
         self.drag.clear();
         if self.anim != null { self.anim.init(); }
         if self.fx != null { self.fx.clear(); }
+        self.motion.reset_landings();
     }
 }