Complete REGULAR_BATTLE immediately, fix stale saveslot tests

REGULAR_BATTLE previously never completed on its own, hanging any
cutscene that reached it - START_BATTLE already waits out the whole
encounter and returns to the overworld before this item ever runs, so
it's now a completes-immediately hook point for future regular-battle-
specific follow-up.

Also updates test_saveslot.c for the mapName/partyMembers schema
additions: the round-trip test now writes a valid map name for its
in-use slot, "missingFieldsUseDefaults" is replaced with a test for the
blank-slot short-circuit it was actually hitting, and a new regression
test locks in that an in-use slot with no map name is treated as corrupt.

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

src/dusk/rpg/cutscene/item/battle/cutsceneregularbattle.c

@@ -11,5 +11,5 @@ bool_t cutsceneRegularBattleUpdate(
   const cutsceneitem_t *item,
   cutsceneitemdata_t *data
 ) {
-  return false;
+  return true;
 }

src/dusk/rpg/cutscene/item/battle/cutsceneregularbattle.h

@@ -9,12 +9,15 @@
 #include "rpg/cutscene/item/cutsceneitembase.h"
 
 /**
- * Updates a regular-battle step. Currently a placeholder - behaves exactly
- * like IDLE (see cutsceneidle.h) and never completes on its own.
+ * Updates a regular-battle step. By the time this item runs, the preceding
+ * START_BATTLE item has already waited out the whole encounter, synced
+ * party HP/MP, and returned to the overworld scene - so this is currently
+ * just a completes-immediately hook point for whatever regular-battle-
+ * specific follow-up (loot, post-battle dialogue, etc.) gets added later.
  *
  * @param item The cutscene item.
  * @param data Runtime data storage.
- * @returns false always.
+ * @returns true always.
  */
 bool_t cutsceneRegularBattleUpdate(
   const cutsceneitem_t *item,