feat: capture data-only battle checkpoints

src/core/Checkpoint.lua

@@ -5,6 +5,7 @@ local SaveSerializer = require("src.core.SaveSerializer")
 local SaveData = require("src.core.SaveData")
 local Version = require("src.core.Version")
 local BattleState = require("src.battle.BattleState")
+local BattleCheckpoint = require("src.core.BattleCheckpoint")
 
 local Checkpoint = {}
 
@@ -172,6 +173,25 @@ function Checkpoint.capture(game)
       .. tostring(err)
   end
 
+  if capability.kind == "battle" then
+    local battle = game.stack:top()
+    local runtime, rngOrCode, battleMessage =
+      BattleCheckpoint.capture(game, battle, progress, dataCopy)
+    if not runtime then return nil, rngOrCode, battleMessage end
+    return {
+      format = Checkpoint.FORMAT,
+      kind = "battle",
+      identity = {
+        engineVersion = Version.engine,
+        gameVersion = game.save.version,
+        playthroughId = game.save.meta.playthroughId,
+      },
+      save = progress,
+      runtime = runtime,
+      rng = rngOrCode,
+    }
+  end
+
   local player = game.overworld.player
   return {
     format = Checkpoint.FORMAT,