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_view.sx

@@ -400,6 +400,12 @@ impl View for BoardView {
         self.compute_layout(frame);
         if event.* == {
             case .mouse_down: (d) {
+                // Gate input at gesture START: while a move animation is in
+                // flight the board ignores new gestures for the WHOLE in-flight
+                // window, so a press begun mid-animation never latches a drag and
+                // so can't commit when the animation later ends. The press is
+                // still consumed; input resumes once the timeline settles.
+                if !accepts_input(self.anim) { return true; }
                 self.drag.begin(d.position);
                 return true;
             }
@@ -407,9 +413,6 @@ impl View for BoardView {
                 if !self.drag.active { return false; }
                 start := self.drag.start;
                 self.drag.clear();
-                // Ignore swipes while a move is still animating so two timelines
-                // never overlap; the model is already settled by then either way.
-                if self.anim != null and self.anim.active { return true; }
                 if intent := swipe_intent(@self.layout, start, d.position) {
                     mv := plan_and_commit(self.board, intent.a, intent.b);
                     if self.anim != null { self.anim.begin(mv); }