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/stage/manifest.rs

@@ -428,6 +428,7 @@ impl StageCameraProfile {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::stage::line::StageLineID;
 
     fn platform(ledges: Vec<StageLedgeDefinition>) -> StagePolyDefinition {
         StagePolyDefinition {
@@ -461,6 +462,35 @@ mod tests {
         assert_eq!(ledges[1].position, FGVec2::lit("5", "0"));
     }
 
+    #[test]
+    fn ledge_ground_segments_point_toward_the_stage_interior() {
+        let definition = platform(vec![
+            StageLedgeDefinition {
+                vertex: 0,
+                hang_side: StageLedgeSide::Left,
+            },
+            StageLedgeDefinition {
+                vertex: 1,
+                hang_side: StageLedgeSide::Right,
+            },
+        ]);
+        let collision = StageCollision {
+            stage_polys: vec![StagePoly::build(definition.poly_type, &definition.lines)],
+            ledges: definition.resolved_ledges(0).collect(),
+        };
+
+        for vertex in [0, 1] {
+            let (line_id, segment) = collision
+                .find_ledge_ground_segment(StageVertexID { polygon: 0, vertex })
+                .unwrap();
+            assert_eq!(line_id, StageLineID { polygon: 0, segment: 0 });
+            assert!(matches!(
+                segment.segment_type,
+                StagePolyLineSegmentType::Floor
+            ));
+        }
+    }
+
     #[test]
     fn invalid_and_duplicate_ledge_vertices_are_rejected() {
         let missing = platform(vec![StageLedgeDefinition {