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/turn.rs

@@ -1,13 +1,11 @@
 use super::{CollisionContext, EnterContext, FighterStateImpl, InterruptContext, UpdateContext};
 use crate::fighter::animation::AnimKind;
-use crate::fighter::state::ground::{self, GroundedStateCommon};
+use crate::fighter::state::ground;
 use crate::fighter::state::{FighterState, dash, jump, wait, walk};
 use crate::input::FighterCommands;
 
 #[derive(Debug, Clone, Hash)]
-pub struct TurnState {
-    pub grounded_common: GroundedStateCommon,
-}
+pub struct TurnState;
 
 impl FighterStateImpl for TurnState {
     const NAME: &'static str = "Turn";
@@ -19,19 +17,19 @@ impl FighterStateImpl for TurnState {
                 .input_common
                 .turn_into_dashback_window as u32
         {
-            if let Some(state) = dash::check_input(state_context, &self.grounded_common) {
+            if let Some(state) = dash::check_input(state_context) {
                 return Some(state);
             }
         }
-        if let Some(state) = jump::check_input(state_context, &self.grounded_common) {
+        if let Some(state) = jump::check_input(state_context) {
             *state_context.facing_direction = state_context.facing_direction.reverse();
             Some(state)
         } else if state_context.is_current_animation_finished() {
-            if let Some(state) = dash::check_input(state_context, &self.grounded_common) {
+            if let Some(state) = dash::check_input(state_context) {
                 Some(state)
-            } else if let Some(state) = walk::check_input(state_context, &self.grounded_common) {
+            } else if let Some(state) = walk::check_input(state_context) {
                 Some(state)
-            } else if let Some(state) = wait::check_input(state_context, &self.grounded_common) {
+            } else if let Some(state) = wait::check_input(state_context) {
                 Some(state)
             } else {
                 None
@@ -54,20 +52,15 @@ impl FighterStateImpl for TurnState {
         &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,
-    grounded_common: &GroundedStateCommon,
-) -> Option<FighterState> {
+pub fn check_input(state_context: &InterruptContext) -> Option<FighterState> {
     let input_frame = state_context.input.get_last_frame();
 
     let has_smash_input = state_context
@@ -81,9 +74,7 @@ pub fn check_input(
         && input_frame.movement.x.signum() != state_context.facing_direction.to_sign()
         && !has_smash_input
     {
-        return Some(FighterState::Turn(TurnState {
-            grounded_common: grounded_common.clone(),
-        }));
+        return Some(FighterState::Turn(TurnState));
     }
     None
 }