Fix fighter visuals after landing snap (#67)

## Summary

- Mark landing position corrections as presentation discontinuities.
- Render and frame fighters from the current position for the snap frame.
- Keep previous translation history unchanged for gameplay collision and attack logic.

Reviewed-on: #67
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/motion.rs

@@ -21,6 +21,11 @@ pub struct FighterVelocity(pub FGVec2);
 #[derive(Component, Deref, DerefMut, Default, Clone, Copy, Hash)]
 pub struct FighterPreviousTranslation(pub FGVec2);
 
+/// Marks a simulation frame whose position changed discontinuously for
+/// presentation purposes (for example, a landing correction).
+#[derive(Component, Default, Clone, Copy, Hash, Debug, PartialEq, Eq)]
+pub struct FighterPresentationSnap(pub bool);
+
 #[derive(Component, Deref, DerefMut, Default, Clone, Copy, Hash)]
 #[require(FighterPreviousTranslation)]
 pub struct FighterTranslation(pub FGVec2);
@@ -52,10 +57,31 @@ pub fn sample_presentation_translation(
     }
 }
 
+pub fn sample_presentation_translation_with_snap(
+    previous: FighterPreviousTranslation,
+    current: FighterTranslation,
+    overstep_fraction: f32,
+    mode: PresentationMotionMode,
+    snapped: FighterPresentationSnap,
+) -> Vec2 {
+    if snapped.0 {
+        Vec2::new(current.x.to_num(), current.y.to_num())
+    } else {
+        sample_presentation_translation(previous, current, overstep_fraction, mode)
+    }
+}
+
+pub fn clear_fighter_presentation_snaps(mut fighters: Query<&mut FighterPresentationSnap>) {
+    for mut snap in &mut fighters {
+        snap.0 = false;
+    }
+}
+
 pub fn apply_fighter_translation_to_visuals(
     query: Query<(
         &FighterTranslation,
         &FighterPreviousTranslation,
+        &FighterPresentationSnap,
         &FighterHitstopShake,
         &mut Transform,
     )>,
@@ -64,12 +90,13 @@ pub fn apply_fighter_translation_to_visuals(
     game_settings: Res<GameSettings>,
 ) {
     let overstep_fraction = timing.overstep_fraction();
-    for (translation, previous_translation, hitstop_shake, mut transform) in query {
-        let translation = sample_presentation_translation(
+    for (translation, previous_translation, presentation_snap, hitstop_shake, mut transform) in query {
+        let translation = sample_presentation_translation_with_snap(
             *previous_translation,
             *translation,
             overstep_fraction,
             debug_settings.motion_sampling,
+            *presentation_snap,
         );
         transform.translation = Vec3::new(
             translation.x,
@@ -169,6 +196,19 @@ mod tests {
         assert_eq!(position, Vec2::new(7.0, 14.0));
     }
 
+    #[test]
+    fn presentation_translation_uses_current_sample_after_snap() {
+        let position = sample_presentation_translation_with_snap(
+            FighterPreviousTranslation(FGVec2::new(FGi32::lit("2"), FGi32::lit("4"))),
+            FighterTranslation(FGVec2::new(FGi32::lit("6"), FGi32::lit("12"))),
+            0.25,
+            PresentationMotionMode::Extrapolation,
+            FighterPresentationSnap(true),
+        );
+
+        assert_eq!(position, Vec2::new(6.0, 12.0));
+    }
+
     #[test]
     fn snapshot_fighter_motion_copies_current_world_state() {
         let mut world = World::new();