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/debug_tools/save_state.rs

@@ -1,12 +1,22 @@
 use bevy::prelude::*;
 
 use crate::{
-    camera::MatchCamera, fighter::{
-        FighterECB, FighterFacingDirection, FighterHitboxes, FighterHitstop, FighterHitstopShake, FighterPreviousFacingDirection, FighterPreviousTranslation, FighterTranslation, FighterVelocity, Grounded, animation::FighterAnimationFrame, attack::{
+    camera::MatchCamera,
+    fighter::{
+        FighterECB, FighterFacingDirection, FighterHitboxes, FighterHitstop, FighterHitstopShake,
+        FighterPreviousFacingDirection, FighterPreviousTranslation, FighterSurfaceState,
+        FighterTranslation, FighterVelocity,
+        animation::FighterAnimationFrame,
+        attack::{
             FighterDamage, FighterKnockbackVelocity, FighterPendingHitReaction, FighterPendingHits,
             FighterSolvedHurtboxes, FighterStaleMoveQueue,
-        }, baked_animation::FighterBoneMatrices, state::FighterState,
-    }, input::FighterInput, player::Player, stage::line::StageCollision,
+        },
+        baked_animation::FighterBoneMatrices,
+        state::FighterState,
+    },
+    input::FighterInput,
+    player::Player,
+    stage::line::StageCollision,
 };
 
 #[derive(Resource, Default)]
@@ -44,7 +54,7 @@ struct FighterCheckpoint {
     hitboxes: FighterHitboxes,
     solved_hurtboxes: FighterSolvedHurtboxes,
     bone_matrices: FighterBoneMatrices,
-    grounded: bool,
+    surface: FighterSurfaceState,
 }
 
 pub(super) fn save_or_restore(world: &mut World) {
@@ -143,11 +153,7 @@ pub(super) fn save_or_restore(world: &mut World) {
         entity.insert(saved.hitboxes.clone());
         entity.insert(saved.solved_hurtboxes.clone());
         entity.insert(saved.bone_matrices.clone());
-        if saved.grounded {
-            entity.insert(Grounded);
-        } else {
-            entity.remove::<Grounded>();
-        }
+        entity.insert(saved.surface);
     }
     world.insert_resource(checkpoint.pending_hits.clone());
     world.insert_resource(checkpoint.stage_collision);
@@ -181,7 +187,7 @@ fn capture_fighter(world: &World, entity: Entity) -> Option<FighterCheckpoint> {
         hitboxes: fighter.get::<FighterHitboxes>()?.clone(),
         solved_hurtboxes: fighter.get::<FighterSolvedHurtboxes>()?.clone(),
         bone_matrices: fighter.get::<FighterBoneMatrices>()?.clone(),
-        grounded: fighter.contains::<Grounded>(),
+        surface: *fighter.get::<FighterSurfaceState>()?,
     })
 }