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.
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