Implement Melee-style knockback stacking (#97)

## Summary

- stack residual airborne knockback per axis after the configured launch-frame threshold
- apply DI after combining the launch vectors while retaining the second hit's hitstun
- preserve and decay residual knockback after hitstun
- make stacking state rollback-safe and include it in debug save states
- clear launch state on landing, ledge interactions, KO, and respawn

## Testing

- `cargo check --tests`
- `cargo test` (159 tests passed)
- `git diff --check`

## Asset configuration

For Melee timing, `knockback_stacking_window` should be set to `10`. The local ignored asset package has been updated accordingly.

Reviewed-on: #97
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/state/mod.rs

@@ -7,8 +7,8 @@ use bevy::prelude::*;
 
 use crate::fighter::animation::{AnimKind, FighterAnimationFrame};
 use crate::fighter::attack::{
-    FighterAttackScriptAssets, FighterKnockbackVelocity, FighterPendingHitReaction,
-    FighterStaleMoveQueue, HitStop,
+    FighterAttackScriptAssets, FighterKnockbackState, FighterKnockbackVelocity,
+    FighterPendingHitReaction, FighterStaleMoveQueue, HitStop,
 };
 use crate::fighter::attributes::FighterAttributes;
 use crate::fighter::baked_animation::{BakedFighterAnimations, FighterBoneMatrices};
@@ -167,6 +167,7 @@ pub struct EnterContext<'a> {
     pub velocity: &'a mut FighterVelocity,
     pub translation: &'a mut FighterTranslation,
     pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub stale_move_queue: &'a mut FighterStaleMoveQueue,
     pub facing_direction: &'a mut FighterFacingDirection,
     pub animation_frame: &'a mut FighterAnimationFrame,
@@ -204,6 +205,8 @@ impl EnterContext<'_> {
 pub struct ExitContext<'a> {
     pub input: &'a mut FighterInput,
     pub hitboxes: &'a mut FighterHitboxes,
+    pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub surface: &'a mut FighterSurfaceState,
     pub vulnerability: &'a mut FighterInvulnerabilityState,
 }
@@ -216,6 +219,7 @@ pub struct UpdateContext<'a> {
     pub velocity: &'a mut FighterVelocity,
     pub surface: &'a mut FighterSurfaceState,
     pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub vulnerability: &'a mut FighterInvulnerabilityState,
     pub ledge_info: &'a mut FighterLedgeInformation,
     pub game_settings: &'a GameSettings,
@@ -251,6 +255,8 @@ pub struct CollisionContext<'a> {
     pub prev_translation: &'a FighterPreviousTranslation,
     pub input: &'a FighterInput,
     pub velocity: &'a mut FighterVelocity,
+    pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub surface: &'a mut FighterSurfaceState,
     pub ecb: &'a mut FighterECB,
     pub desired_ecb: &'a mut FighterDesiredECB,
@@ -335,6 +341,7 @@ pub struct FighterFrameQuery {
     pub velocity: &'static mut FighterVelocity,
     pub surface: &'static mut FighterSurfaceState,
     pub knockback_velocity: &'static mut FighterKnockbackVelocity,
+    pub knockback_state: &'static mut FighterKnockbackState,
     pub stale_move_queue: &'static mut FighterStaleMoveQueue,
     pub ecb: &'static mut FighterECB,
     pub desired_ecb: &'static mut FighterDesiredECB,
@@ -403,6 +410,7 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
             hitboxes: &mut self.hitboxes,
             velocity: &mut self.velocity,
             knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             translation: &mut self.translation,
             stale_move_queue: &mut self.stale_move_queue,
             facing_direction: &mut self.facing_direction,
@@ -424,6 +432,8 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
         ExitContext {
             input: &mut self.input,
             hitboxes: &mut self.hitboxes,
+            knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             surface: &mut self.surface,
             vulnerability: &mut self.vulnerability,
         }
@@ -453,6 +463,7 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
             velocity: &mut self.velocity,
             surface: &mut self.surface,
             knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             game_settings,
             animation_root_motion_delta,
             animation_root_position,
@@ -483,6 +494,8 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
             presentation_snap: &mut self.presentation_snap,
             input: &self.input,
             velocity: &mut self.velocity,
+            knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             surface: &mut self.surface,
             ecb: &mut self.ecb,
             desired_ecb: &mut self.desired_ecb,
@@ -655,9 +668,16 @@ pub fn state_interrupt_system(
             }
 
             info!("Entering hitstop kind: {:?}", hitstop_kind);
+            let stack_previous_knockback = !frame.surface.is_grounded()
+                && !frame.knockback_velocity.0.length_squared().is_zero()
+                && frame
+                    .knockback_state
+                    .can_stack(game_settings.fighter_common.knockback_stacking_window);
+
             Some(FighterState::Hitstun(HitstunState::new(
                 reaction,
                 hitstop_kind,
+                stack_previous_knockback,
             )))
         } else if hitstop.remaining_frames > 0 {
             None
@@ -753,6 +773,8 @@ pub fn state_collision_interrupt_system(
 
             // Clear hitstop
             hitstop.remaining_frames = 0;
+            frame.knockback_velocity.0 = FGVec2::ZERO;
+            frame.knockback_state.clear();
 
             // subtract one stock
 
@@ -846,6 +868,8 @@ mod tests {
         let input = FighterInput::default();
         let mut translation = FighterTranslation::default();
         let mut velocity = FighterVelocity::default();
+        let mut knockback_velocity = FighterKnockbackVelocity::default();
+        let mut knockback_state = FighterKnockbackState::default();
         let mut surface = FighterSurfaceState::default();
         let mut ecb = FighterECB::default();
         let mut desired_ecb = FighterDesiredECB {
@@ -865,6 +889,8 @@ mod tests {
             prev_translation: &previous_translation,
             input: &input,
             velocity: &mut velocity,
+            knockback_velocity: &mut knockback_velocity,
+            knockback_state: &mut knockback_state,
             surface: &mut surface,
             ecb: &mut ecb,
             desired_ecb: &mut desired_ecb,