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/air.rs

@@ -1,3 +1,6 @@
+use crate::fighter::state::ground::GroundedStateCommon;
+use crate::fighter::state::land::{LandingState, LandingType};
+use crate::fighter::state::{CollisionContext, FighterState};
 use crate::fighter::*;
 
 pub fn integrate_gravity(velocity: &mut FighterVelocity, attributes: &FighterAttributes) {
@@ -53,6 +56,27 @@ pub fn apply_air_motion(
     translation.0 += velocity.0;
 }
 
+pub fn update_standard_motion(ctx: &mut crate::fighter::state::UpdateContext) {
+    integrate_gravity(ctx.velocity, ctx.fighter_attributes);
+    apply_air_drift(ctx.velocity, ctx.fighter_attributes, ctx.input);
+    apply_air_motion(ctx.velocity, ctx.translation, ctx.prev_translation);
+}
+
+pub fn collision_interrupt(
+    ctx: &mut CollisionContext,
+    landing_type: LandingType,
+) -> Option<FighterState> {
+    let res = ctx.air_collide_with_stage()?;
+    ctx.translation.0 = res.hit_position - ctx.ecb.get_bottom_point();
+    let grounded_common = GroundedStateCommon {
+        current_line_id: res.line_id,
+    };
+    Some(FighterState::Land(LandingState::create(
+        grounded_common,
+        landing_type,
+    )))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;