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/ground.rs

@@ -248,6 +248,7 @@ mod tests {
     use crate::fighter::ecb::FighterPreviousECB;
     use crate::fighter::{FighterECB, FighterSurfaceState};
     use crate::stage::line::StagePolyType;
+    use crate::input::FighterInput;
 
     fn two_segment_floor() -> StagePoly {
         StagePoly::build(
@@ -378,12 +379,14 @@ mod tests {
         let mut context = CollisionContext {
             translation: &mut translation,
             prev_translation: &previous_translation,
+            input: &FighterInput::default(),
             velocity: &mut velocity,
             surface: &mut surface,
             ecb: &mut ecb,
             prev_ecb: &previous_ecb,
             stage_collision: &stage_collision,
             leave_ground_ecb_lock_duration: 10,
+            downward_stick_threshold: FGi32::ONE,
         };
 
         assert!(matches!(
@@ -429,12 +432,14 @@ mod tests {
         let mut context = CollisionContext {
             translation: &mut translation,
             prev_translation: &previous_translation,
+            input: &FighterInput::default(),
             velocity: &mut velocity,
             surface: &mut surface,
             ecb: &mut ecb,
             prev_ecb: &previous_ecb,
             stage_collision: &stage_collision,
             leave_ground_ecb_lock_duration: 10,
+            downward_stick_threshold: FGi32::ONE,
         };
 
         assert!(matches!(