Allow airborne fighters to pass through platforms while holding down (#65)

## Summary
- Ignore soft-platform collision while an airborne fighter holds down.
- Preserve existing momentum and collision with solid floors.
- Add regression coverage for soft and solid platform behavior.

Reviewed-on: #65
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/state/mod.rs

@@ -193,12 +193,14 @@ impl UpdateContext<'_> {
 pub struct CollisionContext<'a> {
     pub translation: &'a mut FighterTranslation,
     pub prev_translation: &'a FighterPreviousTranslation,
+    pub input: &'a FighterInput,
     pub velocity: &'a mut FighterVelocity,
     pub surface: &'a mut FighterSurfaceState,
     pub ecb: &'a mut FighterECB,
     pub prev_ecb: &'a FighterPreviousECB,
     pub stage_collision: &'a StageCollision,
     pub leave_ground_ecb_lock_duration: u32,
+    pub downward_stick_threshold: FGi32,
 }
 
 impl CollisionContext<'_> {
@@ -206,6 +208,8 @@ impl CollisionContext<'_> {
         &self,
     ) -> Option<crate::fighter::collision::AirCollisionWithStageResult> {
         let ignored_polys = self.surface.ignored_polygon_ids();
+        let ignore_soft_platforms = self.input.get_last_frame().movement.y
+            <= -self.downward_stick_threshold;
         crate::fighter::collision::air_collide_with_stage(
             self.prev_translation,
             self.translation,
@@ -213,6 +217,7 @@ impl CollisionContext<'_> {
             self.ecb,
             self.stage_collision,
             &ignored_polys,
+            ignore_soft_platforms,
         )
     }
 }
@@ -349,12 +354,14 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
         CollisionContext {
             translation: &mut self.translation,
             prev_translation: &self.prev_translation,
+            input: &self.input,
             velocity: &mut self.velocity,
             surface: &mut self.surface,
             ecb: &mut self.ecb,
             prev_ecb: &self.prev_ecb,
             stage_collision,
-            leave_ground_ecb_lock_duration: game_settings.fighter_common.leave_ground_ecb_frame_lock_duration
+            leave_ground_ecb_lock_duration: game_settings.fighter_common.leave_ground_ecb_frame_lock_duration,
+            downward_stick_threshold: game_settings.input_common.run_stick_threshold,
         }
     }
 }