Deduplicate fighter state lifecycle policies (#36)

Fixes #22

Extracts shared grounded collision/friction and airborne movement/landing policies while preserving state-specific behavior. Running fighters now pass `can_walk_off = true` so they can run off platform edges. Includes grounded and airborne hitstun call sites.

Reviewed-on: #36
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/state/dash.rs

@@ -5,8 +5,7 @@ use super::{
 use crate::fighter::FighterFacingDirection;
 use crate::fighter::animation::AnimKind;
 use crate::fighter::state::FighterState;
-use crate::fighter::state::fall::FallState;
-use crate::fighter::state::ground::{GroundedMotionResult, GroundedStateCommon};
+use crate::fighter::state::ground::GroundedStateCommon;
 use crate::input::FighterCommands::{SmashMoveLeft, SmashMoveRight};
 
 #[derive(Debug, Clone, Hash)]
@@ -107,13 +106,7 @@ impl FighterStateImpl for DashState {
         &mut self,
         state_context: &mut CollisionContext,
     ) -> Option<FighterState> {
-        if let GroundedMotionResult::InAir =
-            ground::collide_with_stage_grounded(state_context, &mut self.ground_common, true)
-        {
-            Some(FighterState::Fall(FallState))
-        } else {
-            None
-        }
+        ground::collision_interrupt(state_context, &mut self.ground_common, true)
     }
 }