P6.1: lock input for the full in-flight animation window

A swipe that began while a move animation was playing could still commit:
mouse_down latched the drag unconditionally and the animation-active check
sat at mouse_up, so a press made mid-animation committed once the timeline
finished before release — against a board mid-transition.

Gate input at gesture START instead. Add a pure `accepts_input(anim)`
predicate (false while a timeline is active) and check it at mouse_down: a
press begun mid-animation is dropped and never latches a drag, so it cannot
commit when the animation later settles. The now-dead mouse_up gate is
removed. Animation visuals and the logical model are unchanged.

Extend tests/anim_plan.sx to assert accepts_input rejects for the whole
window (idle accept / busy reject / settled accept) and that press-gating
drops the exact failure gesture a release-gate would let through.

tests/expected/anim_plan.stdout

@@ -14,4 +14,7 @@ YOYYROBB
 OROBPPRB
 == illegal swap: untouched ==
 legal false rounds 0 score 0 moves 0
+== input gate: locked while animating ==
+accepts idle true busy false settled true
+release_gate_commits true press_gate_commits false
 ok: animation layer leaves the model result unchanged