Initial work on rollback and refactor

src/player.rs

@@ -0,0 +1,14 @@
+use bevy::prelude::*;
+use bevy_ggrs::prelude::*;
+
+// #[require(Rollback)] is the idiomatic way to ensure an entity is always included in
+// the rollback system. Whenever a Player is spawned, Rollback (and its RollbackId) will
+// be added automatically — no need to add it manually in the spawn bundle.
+/// Ties an entity to one of the session's input streams. Lives at the crate
+/// root rather than in `netcode` because "which player is this" is a game
+/// concept; `netcode` only decides where the inputs come from.
+#[derive(Default, Component)]
+#[require(Rollback)]
+pub struct Player {
+    pub handle: usize,
+}