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.

board_anim.sx

@@ -184,3 +184,13 @@ BoardAnim :: struct {
         AnimPhase.{ kind = .done, round = 0, t = 1.0 }
     }
 }
+
+// Input gate: the board accepts a new swipe/tap gesture only when no move
+// animation is in flight. The view checks this at gesture START (mouse_down),
+// not at commit (mouse_up), so a gesture begun while a timeline is playing never
+// latches a drag and so cannot commit when the animation later settles. Input
+// resumes once `tick` clears `active` at the end of the timeline. A null anim
+// (no animation layer wired) always accepts.
+accepts_input :: (anim: *BoardAnim) -> bool {
+    anim == null or !anim.active
+}