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/fall.rs

@@ -1,7 +1,5 @@
 use super::{CollisionContext, EnterContext, FighterStateImpl, InterruptContext, UpdateContext};
 use crate::fighter::animation::AnimKind;
-use crate::fighter::state::ground::GroundedStateCommon;
-use crate::fighter::state::land::{LandingState, LandingType};
 use crate::fighter::state::{FighterState, air, jump};
 
 #[derive(Debug, Clone, Copy, Hash)]
@@ -18,36 +16,17 @@ impl FighterStateImpl for FallState {
     }
 
     fn update(&mut self, state_context: &mut UpdateContext) {
-        air::integrate_gravity(state_context.velocity, state_context.fighter_attributes);
-        air::apply_air_drift(
-            state_context.velocity,
-            state_context.fighter_attributes,
-            state_context.input,
-        );
-        air::apply_air_motion(
-            state_context.velocity,
-            state_context.translation,
-            state_context.prev_translation,
-        );
+        air::update_standard_motion(state_context);
     }
 
     fn check_collision_interrupt(
         &mut self,
         state_context: &mut CollisionContext,
     ) -> Option<FighterState> {
-        if let Some(res) = state_context.air_collide_with_stage() {
-            // Adjust translation
-            state_context.translation.0 = res.hit_position - state_context.ecb.get_bottom_point();
-            let grounded_common = GroundedStateCommon {
-                current_line_id: res.line_id,
-            };
-            Some(FighterState::Land(LandingState::create(
-                grounded_common,
-                LandingType::Normal,
-            )))
-        } else {
-            None
-        }
+        air::collision_interrupt(
+            state_context,
+            crate::fighter::state::land::LandingType::Normal,
+        )
     }
 }