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/mod.rs

@@ -18,6 +18,7 @@ pub mod hurtbox;
 pub mod manifest;
 pub mod motion;
 pub mod state;
+pub mod surface;
 pub mod visual;
 
 // The fighter components are re-exported so the rest of the crate can say
@@ -28,18 +29,29 @@ pub use attributes::FighterAttributes;
 pub use ecb::FighterECB;
 pub use manifest::FighterId;
 pub use motion::{
-    FighterPreviousFacingDirection, FighterPreviousTranslation, FighterTranslation,
-    FighterVelocity, Grounded,
+    FighterPreviousFacingDirection, FighterPreviousTranslation, FighterTranslation, FighterVelocity,
 };
+pub use surface::FighterSurfaceState;
 
 use crate::{
-    AppState, fighter::{
+    AppState,
+    fighter::{
         attack::{
             AttackKind, FighterAttackPlugin, FighterAttackScriptAssets, FighterDamage,
             FighterKnockbackVelocity, FighterPendingHitReaction, FighterSolvedHurtboxes,
             FighterStaleMoveQueue,
-        }, baked_animation::{FighterBoneIndexMap, FighterBoneMatrices}, manifest::FighterManifest, state::{FighterState, air::AirborneStateCommon, fall::FallState}, visual::FighterAnimations,
-    }, input::FighterInput, math::{int::FGi32, vec::FGVec2}, player::Player, schedule::GameplaySet, scripting::FighterAttackScript, stage::line::StageCollision,
+        },
+        baked_animation::{FighterBoneIndexMap, FighterBoneMatrices},
+        manifest::FighterManifest,
+        state::{FighterState, air::AirborneStateCommon, fall::FallState},
+        visual::FighterAnimations,
+    },
+    input::FighterInput,
+    math::{int::FGi32, vec::FGVec2},
+    player::Player,
+    schedule::GameplaySet,
+    scripting::FighterAttackScript,
+    stage::line::StageCollision,
 };
 use state::state_interrupt_system;
 
@@ -58,14 +70,14 @@ pub enum FighterFacingDirection {
 pub enum FighterHitstopShakeKind {
     #[default]
     Horizontal,
-    Vertical
+    Vertical,
 }
 
 #[derive(Component, Copy, Clone, Hash, Debug, PartialEq, Reflect, Default)]
 pub enum FighterHitstopShakeRole {
     #[default]
     Attacker,
-    Receiver
+    Receiver,
 }
 
 #[derive(Component, Default, Copy, Clone, Hash)]
@@ -73,7 +85,7 @@ pub struct FighterHitstopShake {
     pub remaining_frames: u32,
     pub total_frames: u32,
     pub kind: FighterHitstopShakeKind,
-    pub role: FighterHitstopShakeRole
+    pub role: FighterHitstopShakeRole,
 }
 
 #[derive(Component, Default, Copy, Clone, Hash)]
@@ -133,7 +145,7 @@ pub struct FighterVisual;
     FighterPendingHitReaction,
     FighterHitstop,
     FighterKnockbackVelocity,
-    FighterTranslation,
+    FighterTranslation
 )]
 pub struct Fighter {
     pub manifest: Handle<FighterManifest>,
@@ -200,8 +212,8 @@ impl Plugin for FighterPlugin {
             .checksum_component_with_hash::<FighterPreviousFacingDirection>()
             .rollback_component_with_copy::<animation::FighterAnimationFrame>()
             .checksum_component_with_hash::<animation::FighterAnimationFrame>()
-            .rollback_component_with_copy::<Grounded>()
-            .checksum_component_with_hash::<Grounded>()
+            .rollback_component_with_copy::<FighterSurfaceState>()
+            .checksum_component_with_hash::<FighterSurfaceState>()
             .rollback_component_with_copy::<FighterFacingDirection>()
             .rollback_component_with_copy::<FighterDamage>()
             .checksum_component_with_hash::<FighterDamage>()
@@ -259,7 +271,7 @@ pub struct FighterActiveAttack {
     pub active_hitboxes: Vec<usize>,
     pub solved_hitboxes: Vec<attack::FighterSolvedHitbox>,
     pub hit_fighters: Vec<Entity>,
-    pub stalled: bool
+    pub stalled: bool,
 }
 
 #[derive(Component, Clone, Debug, Default, Hash)]
@@ -284,6 +296,7 @@ pub fn spawn_fighter(
             Fighter { manifest: manifest },
             FighterFacingDirection::Right,
             FighterPreviousFacingDirection(FighterFacingDirection::Right),
+            FighterSurfaceState::Airborne,
             Transform::from_scale(Vec3::splat(model_scale.to_num())),
             (
                 FighterECB {