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/stage/line.rs

@@ -26,7 +26,7 @@ pub enum StagePolyType {
     Platform,
 }
 
-#[derive(Reflect, Hash, Debug, Clone, Copy, Default)]
+#[derive(Reflect, Hash, Debug, Clone, Copy, Default, PartialEq, Eq)]
 pub struct StageLineID {
     pub polygon: usize,
     pub segment: usize,