Add authoritative fighter surface state (#60)

## Summary
- add a rollback-tracked `FighterSurfaceState` as the single source of truth for stage contact
- move active stage line ownership out of individual grounded fighter states
- update landing, airborne motion, ledge departure, hit reactions, and debug save states to maintain surface contact
- add tests for contact lifecycle, connected-segment traversal, and ledge departure

Reviewed-on: #60
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/state/air.rs

@@ -1,4 +1,3 @@
-use crate::fighter::state::ground::GroundedStateCommon;
 use crate::fighter::state::land::{LandingState, LandingType};
 use crate::fighter::state::{CollisionContext, FighterState};
 use crate::fighter::*;
@@ -30,7 +29,7 @@ pub fn process_fast_fall(
 pub fn integrate_gravity(
     velocity: &mut FighterVelocity,
     attributes: &FighterAttributes,
-    fast_fall: bool
+    fast_fall: bool,
 ) {
     velocity.y -= attributes.gravity;
 
@@ -84,7 +83,9 @@ pub fn apply_air_motion(
     velocity: &FighterVelocity,
     translation: &mut FighterTranslation,
     prev_translation: &mut FighterPreviousTranslation,
+    surface: &mut FighterSurfaceState,
 ) {
+    surface.leave_ground();
     // Vertical velocity should be 0 on the ground
     prev_translation.0 = translation.0;
     translation.0 += velocity.0;
@@ -94,10 +95,24 @@ pub fn update_standard_motion(
     ctx: &mut crate::fighter::state::UpdateContext,
     airborne_common: &mut AirborneStateCommon,
 ) {
-    process_fast_fall(airborne_common, ctx.input, &mut ctx.velocity, ctx.fighter_attributes);
-    integrate_gravity(ctx.velocity, ctx.fighter_attributes, airborne_common.is_fast_falling);
+    process_fast_fall(
+        airborne_common,
+        ctx.input,
+        &mut ctx.velocity,
+        ctx.fighter_attributes,
+    );
+    integrate_gravity(
+        ctx.velocity,
+        ctx.fighter_attributes,
+        airborne_common.is_fast_falling,
+    );
     apply_air_drift(ctx.velocity, ctx.fighter_attributes, ctx.input);
-    apply_air_motion(ctx.velocity, ctx.translation, ctx.prev_translation);
+    apply_air_motion(
+        ctx.velocity,
+        ctx.translation,
+        ctx.prev_translation,
+        ctx.surface,
+    );
 }
 
 pub fn collision_interrupt(
@@ -106,13 +121,8 @@ pub fn collision_interrupt(
 ) -> 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,
-    )))
+    ctx.surface.land(res.line_id);
+    Some(FighterState::Land(LandingState::create(landing_type)))
 }
 
 #[cfg(test)]