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/wait.rs

@@ -3,24 +3,18 @@ use super::{
 };
 use crate::fighter::animation::AnimKind;
 use crate::fighter::state::FighterState;
-use crate::fighter::state::ground::GroundedStateCommon;
 
 #[derive(Debug, Clone, Hash)]
-pub struct WaitState {
-    pub grounded_common: GroundedStateCommon,
-}
+pub struct WaitState;
 
 impl FighterStateImpl for WaitState {
     const NAME: &'static str = "Wait";
     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) {
             Some(state)
-        } else if let Some(state) = ground::grounded_movement_standstill_common_interrupts(
-            state_context,
-            &self.grounded_common,
-        ) {
+        } else if let Some(state) =
+            ground::grounded_movement_standstill_common_interrupts(state_context)
+        {
             // Don't transition into yourself
             if let FighterState::Wait(_) = state {
                 None
@@ -44,26 +38,19 @@ impl FighterStateImpl for WaitState {
         &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.abs() < state_context.game_settings.input_common.stick_deadzone {
-        return Some(FighterState::Wait(WaitState {
-            grounded_common: ground_state_common.clone(),
-        }));
+        return Some(FighterState::Wait(WaitState));
     }
     None
 }