Checkpoint game before sim, auto-revert on abort.

An alternative to
https://github.com/dcs-liberation/dcs_liberation/pull/2891 that I ended
up liking much better (I had assumed some part of the UI would fail or
at least look terrible with this approach, but it seems to work quite
well).

On by default now since it's far less frightening than the previous
thing.

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/2735.

game/persistence/savegamebundle.py

@@ -25,6 +25,7 @@ class SaveGameBundle:
     MANUAL_SAVE_NAME = "player.liberation"
     LAST_TURN_SAVE_NAME = "last_turn.liberation"
     START_OF_TURN_SAVE_NAME = "start_of_turn.liberation"
+    PRE_SIM_CHECKPOINT_SAVE_NAME = "pre_sim_checkpoint.liberation"
 
     def __init__(self, bundle_path: Path) -> None:
         self.bundle_path = bundle_path
@@ -58,6 +59,19 @@ class SaveGameBundle:
                 game, self.START_OF_TURN_SAVE_NAME, copy_from=self
             )
 
+    def save_pre_sim_checkpoint(self, game: Game) -> None:
+        """Writes the save file for the state before beginning simulation.
+
+        This save is the state of the game after the player presses "TAKE OFF", but
+        before the fast-forward simulation begins. It is not practical to rewind, but
+        players commonly will want to cancel and continue planning after pressing that
+        button, so we make a checkpoint that we can reload on abort.
+        """
+        with logged_duration("Saving pre-sim checkpoint"):
+            self._update_bundle_member(
+                game, self.PRE_SIM_CHECKPOINT_SAVE_NAME, copy_from=self
+            )
+
     def load_player(self) -> Game:
         """Loads the save manually created by the player via save/save-as."""
         return self._load_from(self.MANUAL_SAVE_NAME)
@@ -70,6 +84,10 @@ class SaveGameBundle:
         """Loads the save automatically created at the end of the last turn."""
         return self._load_from(self.LAST_TURN_SAVE_NAME)
 
+    def load_pre_sim_checkpoint(self) -> Game:
+        """Loads the save automatically created before the simulation began."""
+        return self._load_from(self.PRE_SIM_CHECKPOINT_SAVE_NAME)
+
     def _load_from(self, name: str) -> Game:
         with ZipFile(self.bundle_path) as zip_bundle:
             with zip_bundle.open(name, "r") as save: