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/mod.rs

@@ -41,8 +41,8 @@ use crate::{
     fighter::{
         attack::{
             AttackKind, FighterAttackPlugin, FighterAttackScriptAssets, FighterDamage,
-            FighterKnockbackVelocity, FighterPendingHitReaction, FighterSolvedHurtboxes,
-            FighterStaleMoveQueue,
+            FighterKnockbackState, FighterKnockbackVelocity, FighterPendingHitReaction,
+            FighterSolvedHurtboxes, FighterStaleMoveQueue,
         },
         baked_animation::FighterBoneIndexMap,
         ecb::{FighterDesiredECB, FighterECBBoneIndices},
@@ -212,6 +212,7 @@ pub struct FighterPendingRespawn(bool);
     FighterHitstop,
     FighterLedgeInformation,
     FighterKnockbackVelocity,
+    FighterKnockbackState,
     FighterTranslation,
     FighterInvulnerabilityState,
     FighterInvulnerabilityVisualState
@@ -330,6 +331,8 @@ impl Plugin for FighterPlugin {
             .checksum_component_with_hash::<FighterInvulnerabilityState>()
             .rollback_component_with_copy::<FighterKnockbackVelocity>()
             .checksum_component_with_hash::<FighterKnockbackVelocity>()
+            .rollback_component_with_copy::<FighterKnockbackState>()
+            .checksum_component_with_hash::<FighterKnockbackState>()
             .rollback_component_with_clone::<baked_animation::FighterBoneMatrices>()
             .checksum_component_with_hash::<baked_animation::FighterBoneMatrices>()
             // Debug views.