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/fighter/surface.rs

@@ -0,0 +1,62 @@
+use bevy::prelude::*;
+
+use crate::stage::line::StageLineID;
+
+/// The stage surface currently supporting a fighter.
+///
+/// This is physics state rather than fighter-state-machine state. Keeping it
+/// separate means grounded, landing, and hitstun states all share one source
+/// of truth for stage contact.
+#[derive(Component, Clone, Copy, Debug, Default, Hash, PartialEq, Eq)]
+pub enum FighterSurfaceState {
+    #[default]
+    Airborne,
+    Grounded {
+        line_id: StageLineID,
+    },
+}
+
+impl FighterSurfaceState {
+    pub fn is_grounded(self) -> bool {
+        matches!(self, Self::Grounded { .. })
+    }
+
+    pub fn line_id(self) -> Option<StageLineID> {
+        match self {
+            Self::Airborne => None,
+            Self::Grounded { line_id } => Some(line_id),
+        }
+    }
+
+    pub(crate) fn land(&mut self, line_id: StageLineID) {
+        *self = Self::Grounded { line_id };
+    }
+
+    pub(crate) fn leave_ground(&mut self) {
+        *self = Self::Airborne;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn contact_lifecycle_has_one_source_of_truth() {
+        let mut surface = FighterSurfaceState::default();
+        assert!(!surface.is_grounded());
+        assert_eq!(surface.line_id(), None);
+
+        let line_id = StageLineID {
+            polygon: 2,
+            segment: 3,
+        };
+        surface.land(line_id);
+        assert!(surface.is_grounded());
+        assert_eq!(surface.line_id(), Some(line_id));
+
+        surface.leave_ground();
+        assert!(!surface.is_grounded());
+        assert_eq!(surface.line_id(), None);
+    }
+}