Initial work on rollback and refactor

src/fighter/ecb.rs

@@ -0,0 +1,21 @@
+use bevy::prelude::*;
+
+/// Environment collision box: the diamond used to resolve a fighter against
+/// the stage. Purely geometry — the collision response lives in
+/// [`super::collision`].
+#[derive(Component, Debug, Clone, Copy)]
+#[require(Transform)]
+pub struct FighterECB {
+    pub vertical_half: f32,
+    pub horizontal_half: f32,
+}
+
+impl FighterECB {
+    pub fn to_3d_lineloop(&self, trf: Transform) -> [Vec3; 4] {
+        let rel_up = trf.transform_point(Vec3::new(0.0, self.vertical_half, 0.0));
+        let rel_down = trf.transform_point(Vec3::new(0.0, -self.vertical_half, 0.0));
+        let rel_left = trf.transform_point(Vec3::new(-self.horizontal_half, 0.0, 0.0));
+        let rel_right = trf.transform_point(Vec3::new(self.horizontal_half, 0.0, 0.0));
+        [rel_up, rel_right, rel_down, rel_left]
+    }
+}