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/walk.rs

@@ -4,23 +4,17 @@ use super::{
 };
 use crate::fighter::animation::AnimKind;
 use crate::fighter::state::FighterState;
-use crate::fighter::state::ground::GroundedStateCommon;
 
 #[derive(Debug, Clone, Hash)]
-pub struct WalkState {
-    pub grounded_common: GroundedStateCommon,
-}
+pub struct WalkState;
 
 impl FighterStateImpl for WalkState {
     const NAME: &'static str = "Walk";
     fn check_interrupt(&self, state_context: &mut InterruptContext) -> Option<FighterState> {
-        if let Some(state) =
-            ground::grounded_movement_common_interrupts(state_context, &self.grounded_common)
-        {
+        if let Some(state) = ground::grounded_movement_common_interrupts(state_context) {
             return Some(state);
         }
-        wait::check_input(state_context, &self.grounded_common)
-            .or_else(|| dash::check_input(state_context, &self.grounded_common))
+        wait::check_input(state_context).or_else(|| dash::check_input(state_context))
     }
 
     fn on_enter(&mut self, state_context: &mut EnterContext) {
@@ -54,26 +48,19 @@ impl FighterStateImpl for WalkState {
         &mut self,
         state_context: &mut CollisionContext,
     ) -> Option<FighterState> {
-        ground::collision_interrupt(state_context, &mut self.grounded_common, false)
+        ground::collision_interrupt(state_context, false)
     }
-    
-    fn is_visually_grounded<'a>(
-        &'a self,
-    ) -> bool {
+
+    fn is_visually_grounded<'a>(&'a self) -> bool {
         true
     }
 }
 
-pub fn check_input(
-    state_context: &InterruptContext,
-    ground_state_common: &GroundedStateCommon,
-) -> Option<FighterState> {
+pub fn check_input(state_context: &InterruptContext) -> Option<FighterState> {
     let input_frame = state_context.input.get_last_frame();
 
     if !input_frame.movement.x.is_zero() {
-        return Some(FighterState::Walk(WalkState {
-            grounded_common: ground_state_common.clone(),
-        }));
+        return Some(FighterState::Walk(WalkState));
     }
 
     None