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/ground_attack.rs

@@ -2,14 +2,13 @@ use crate::fighter::{
     attack::{AttackKind, update_active_hitbox_list},
     state::{
         CollisionContext, EnterContext, ExitContext, FighterState, FighterStateImpl,
-        InterruptContext, UpdateContext, ground::GroundedStateCommon, wait::WaitState,
+        InterruptContext, UpdateContext, wait::WaitState,
     },
 };
 
 #[derive(Clone, Debug, Hash)]
 pub struct GroundAttackState {
     pub attack_kind: AttackKind,
-    pub grounded_common: GroundedStateCommon,
 }
 
 impl FighterStateImpl for GroundAttackState {
@@ -17,9 +16,7 @@ impl FighterStateImpl for GroundAttackState {
 
     fn check_interrupt(&self, state_context: &mut InterruptContext) -> Option<FighterState> {
         if state_context.is_current_animation_finished() {
-            Some(FighterState::Wait(WaitState {
-                grounded_common: self.grounded_common.clone(),
-            }))
+            Some(FighterState::Wait(WaitState))
         } else {
             None
         }
@@ -53,7 +50,7 @@ impl FighterStateImpl for GroundAttackState {
             active_hitboxes: Vec::new(),
             solved_hitboxes: Vec::new(),
             hit_fighters: Vec::new(),
-            stalled: false
+            stalled: false,
         });
     }
 
@@ -67,18 +64,13 @@ impl FighterStateImpl for GroundAttackState {
     ) -> Option<FighterState> {
         None
     }
-    
-    fn is_visually_grounded<'a>(
-        &'a self,
-    ) -> bool {
+
+    fn is_visually_grounded<'a>(&'a self) -> bool {
         true
     }
 }
 
-pub fn check_interrupt(
-    state_context: &InterruptContext,
-    grounded_common: &GroundedStateCommon,
-) -> Option<FighterState> {
+pub fn check_interrupt(state_context: &InterruptContext) -> Option<FighterState> {
     if state_context
         .input
         .has_command(crate::input::FighterCommands::Attack)
@@ -89,7 +81,6 @@ pub fn check_interrupt(
             .to_attack_kind_grounded();
         Some(FighterState::GroundAttack(GroundAttackState {
             attack_kind,
-            grounded_common: grounded_common.clone(),
         }))
     } else {
         None