Persist and resume in-progress cutscene via save slot; add species stub

Save slots now track the currently-active cutscene file (defaulting to a
new placeholder new_game.jsonc intro) so loading a save resumes it;
cutsceneSystemStartCutsceneWith persists this on every cutscene start, and
a cutsceneSystemLoad ordering bug (loadedFile stamped after starting) is
fixed so the persisted path is always correct. Also adds a
battlefighterspecies_t/battlespeciesid_t stub for upcoming species data.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

assets/cutscenes/new_game.jsonc

@@ -0,0 +1,11 @@
+{
+  // Placeholder for the new-game intro flow - just shows a modal for now.
+  "items": [
+    {
+      "type": "MODAL",
+      "title": "New Game",
+      "message": "This is a placeholder for the new game intro.",
+      "blocking": true
+    }
+  ]
+}