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/land.rs

@@ -3,7 +3,7 @@ use crate::fighter::{
     state::{
         CollisionContext, EnterContext, FighterState, FighterStateImpl, InterruptContext,
         UpdateContext, dash,
-        ground::{self, GroundedStateCommon, grounded_movement_standstill_common_interrupts},
+        ground::{self, grounded_movement_standstill_common_interrupts},
         walk,
     },
 };
@@ -16,16 +16,12 @@ pub enum LandingType {
 
 #[derive(Debug, Clone, Hash)]
 pub struct LandingState {
-    pub grounded_common: GroundedStateCommon,
     pub landing_type: LandingType,
 }
 
 impl LandingState {
-    pub fn create(grounded_common: GroundedStateCommon, landing_type: LandingType) -> Self {
-        Self {
-            grounded_common,
-            landing_type,
-        }
+    pub fn create(landing_type: LandingType) -> Self {
+        Self { landing_type }
     }
 }
 
@@ -34,14 +30,14 @@ impl FighterStateImpl for LandingState {
 
     fn check_interrupt(&self, state_context: &mut InterruptContext) -> Option<FighterState> {
         if state_context.is_current_animation_finished() {
-            grounded_movement_standstill_common_interrupts(state_context, &self.grounded_common)
+            grounded_movement_standstill_common_interrupts(state_context)
         } else if state_context.get_current_animation_frame()
             >= state_context.fighter_attributes.landing_iasa
             && let LandingType::Normal = self.landing_type
         {
-            dash::check_input(state_context, &self.grounded_common).or_else(|| {
-                ground::grounded_movement_common_interrupts(state_context, &self.grounded_common)
-                    .or_else(|| walk::check_input(state_context, &self.grounded_common))
+            dash::check_input(state_context).or_else(|| {
+                ground::grounded_movement_common_interrupts(state_context)
+                    .or_else(|| walk::check_input(state_context))
             })
         } else {
             None
@@ -56,7 +52,7 @@ impl FighterStateImpl for LandingState {
         &mut self,
         state_context: &mut CollisionContext,
     ) -> Option<FighterState> {
-        ground::collision_interrupt(state_context, &mut self.grounded_common, true)
+        ground::collision_interrupt(state_context, true)
     }
 
     fn on_enter(&mut self, state_context: &mut EnterContext) {
@@ -69,10 +65,8 @@ impl FighterStateImpl for LandingState {
             }
         };
     }
-    
-    fn is_visually_grounded<'a>(
-        &'a self,
-    ) -> bool {
+
+    fn is_visually_grounded<'a>(&'a self) -> bool {
         true
     }
 }