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/wait.rs

@@ -3,8 +3,7 @@ use super::{
 };
 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;
 
 #[derive(Debug, Clone, Hash)]
 pub struct WaitState {
@@ -38,39 +37,14 @@ impl FighterStateImpl for WaitState {
     }
 
     fn update(&mut self, state_context: &mut UpdateContext) {
-        let friction =
-            if state_context.velocity.x.abs() > state_context.fighter_attributes.max_walk_vel {
-                state_context.fighter_attributes.ground_friction
-                    * state_context
-                        .game_settings
-                        .fighter_common
-                        .ground_friction_over_walk_speed_multiplier
-            } else {
-                state_context.fighter_attributes.ground_friction
-            };
-
-        let ground_velocity = state_context.velocity.x;
-
-        state_context.velocity.x += ground::apply_grounded_friction(friction, ground_velocity);
-
-        ground::apply_grounded_motion(
-            state_context.velocity,
-            state_context.translation,
-            state_context.prev_translation,
-        );
+        ground::update_friction_motion(state_context);
     }
 
     fn check_collision_interrupt(
         &mut self,
         state_context: &mut CollisionContext,
     ) -> Option<FighterState> {
-        if let GroundedMotionResult::InAir =
-            ground::collide_with_stage_grounded(state_context, &mut self.grounded_common, false)
-        {
-            Some(FighterState::Fall(FallState))
-        } else {
-            None
-        }
+        ground::collision_interrupt(state_context, &mut self.grounded_common, false)
     }
 }