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/land.rs

@@ -3,11 +3,7 @@ use crate::fighter::{
     state::{
         CollisionContext, EnterContext, FighterState, FighterStateImpl, InterruptContext,
         UpdateContext, dash,
-        fall::FallState,
-        ground::{
-            self, GroundedMotionResult, GroundedStateCommon,
-            grounded_movement_standstill_common_interrupts,
-        },
+        ground::{self, GroundedStateCommon, grounded_movement_standstill_common_interrupts},
         walk,
     },
 };
@@ -53,39 +49,14 @@ impl FighterStateImpl for LandingState {
     }
 
     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, true)
-        {
-            Some(FighterState::Fall(FallState))
-        } else {
-            None
-        }
+        ground::collision_interrupt(state_context, &mut self.grounded_common, true)
     }
 
     fn on_enter(&mut self, state_context: &mut EnterContext) {