Fix ledge transitions and root motion loops (#91)

## Summary

- resolve each ledge action to the walkable segment on the stage-interior side
- preserve repeating root-motion displacement across every animation cycle
- add regression coverage for left/right ledge segments and multiple root-motion wraps
- Implement ledge intangibility

Reviewed-on: #91
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/state/ledge_wait.rs

@@ -3,9 +3,9 @@ use crate::fighter::animation::AnimKind;
 use crate::fighter::state::air::AirborneStateCommon;
 use crate::fighter::state::fall::FallState;
 use crate::fighter::state::FighterState;
-use crate::fighter::FighterFacingDirection;
 use crate::fighter::state::jump::JumpStateInitiator;
 use crate::fighter::state::ledge_action::{LedgeActionState};
+use crate::fighter::{FighterFacingDirection, FighterInvulnerabilityState};
 use crate::input::FighterCommands;
 use crate::math::vec::FGVec2;
 use crate::stage::line::{StageLedgeSide, StageVertexID};
@@ -58,6 +58,14 @@ impl FighterStateImpl for LedgeWaitState {
     fn on_enter(&mut self, state_context: &mut EnterContext) {
         state_context.velocity.0 = FGVec2::ZERO;
         state_context.knockback_velocity.0 = FGVec2::ZERO;
+        *state_context.vulnerability = FighterInvulnerabilityState::Intangible {
+            frames_remaining: Some(
+                state_context
+                    .game_settings
+                    .fighter_common
+                    .ledge_grab_invulnerability_duration,
+            ),
+        };
         let Some(ledge) = state_context.stage_collision.find_ledge_by_vertex(self.ledge_id) else {
             return;
         };
@@ -127,4 +135,3 @@ fn is_ledge_drop_input(
 
     false
 }
-