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/hitstun.rs

@@ -7,7 +7,7 @@ use crate::{
             InterruptContext, UpdateContext,
             air::{self, AirborneStateCommon},
             fall::FallState,
-            ground::{GroundedStateCommon, collision_interrupt},
+            ground::collision_interrupt,
             land::LandingType,
             wait::WaitState,
         },
@@ -17,7 +17,7 @@ use crate::{
 
 #[derive(Clone, Debug, Hash)]
 pub enum HitstopStateKind {
-    Grounded(GroundedStateCommon),
+    Grounded,
     Airborne,
 }
 
@@ -43,7 +43,7 @@ impl FighterStateImpl for HitstunState {
 
     fn name(&self) -> String {
         let hitstop_kind = match self.hitstop_kind {
-            HitstopStateKind::Grounded(_) => "Grounded",
+            HitstopStateKind::Grounded => "Grounded",
             HitstopStateKind::Airborne => "Airborne",
         };
 
@@ -58,14 +58,10 @@ impl FighterStateImpl for HitstunState {
     fn check_interrupt(&self, state_context: &mut InterruptContext) -> Option<FighterState> {
         if state_context.get_current_animation_frame() > self.hit_reaction.hitstun_frames {
             match &self.hitstop_kind {
-                HitstopStateKind::Grounded(grounded_state_common) => {
-                    Some(FighterState::Wait(WaitState {
-                        grounded_common: grounded_state_common.clone(),
-                    }))
-                }
-                HitstopStateKind::Airborne => {
-                    Some(FighterState::Fall(FallState::new(AirborneStateCommon::default())))
-                }
+                HitstopStateKind::Grounded => Some(FighterState::Wait(WaitState)),
+                HitstopStateKind::Airborne => Some(FighterState::Fall(FallState::new(
+                    AirborneStateCommon::default(),
+                ))),
             }
         } else {
             None
@@ -85,8 +81,9 @@ impl FighterStateImpl for HitstunState {
         }
 
         match self.hitstop_kind {
-            HitstopStateKind::Grounded(_) => {}
+            HitstopStateKind::Grounded => {}
             HitstopStateKind::Airborne => {
+                ctx.surface.leave_ground();
                 air::integrate_gravity(ctx.velocity, ctx.fighter_attributes, false);
             }
         }
@@ -103,7 +100,7 @@ impl FighterStateImpl for HitstunState {
 
     fn on_enter(&mut self, state_context: &mut EnterContext) {
         let animation = match self.hitstop_kind {
-            HitstopStateKind::Grounded(_) => AnimKind::FlinchGround,
+            HitstopStateKind::Grounded => AnimKind::FlinchGround,
             HitstopStateKind::Airborne => AnimKind::FlinchAir,
         };
         state_context.play_animation(animation, false);
@@ -116,9 +113,7 @@ impl FighterStateImpl for HitstunState {
         state_context: &mut CollisionContext,
     ) -> Option<FighterState> {
         match &mut self.hitstop_kind {
-            HitstopStateKind::Grounded(grounded_state_common) => {
-                collision_interrupt(state_context, grounded_state_common, true)
-            }
+            HitstopStateKind::Grounded => collision_interrupt(state_context, true),
             HitstopStateKind::Airborne => {
                 air::collision_interrupt(state_context, LandingType::Normal)
             }
@@ -127,7 +122,7 @@ impl FighterStateImpl for HitstunState {
 
     fn is_visually_grounded<'a>(&'a self) -> bool {
         match self.hitstop_kind {
-            HitstopStateKind::Grounded(_) => true,
+            HitstopStateKind::Grounded => true,
             HitstopStateKind::Airborne => false,
         }
     }