Fix the build after archiving rpg/ui/scene: drop their last live dependents

rpg/ui/scene moved to archive/ earlier left a handful of otherwise-generic
systems with dead references into them:
- asset/loader/chunk existed solely to load rpg overworld map chunks
  (tile_t, worldpos_t) - deleted outright, along with its
  ASSET_LOADER_TYPE_CHUNK entry in assetloader.c/h.
- save/slot wrapped party_t/storyflagval_t/map-name/cutscene-path, which is
  100% rpg save data - deleted outright (save.c/h, savedevice.c/h and the
  Linux save backend keep their generic settings/device-discovery logic,
  just lose the slot read/write pieces that only existed for it).
- locale/localemanager.c and test/CMakeLists.txt/test/save/* had a few
  stray includes and test cases that only existed for the now-gone code.

Also restores assets/locale/en_US.po (copied back from archive/assets/) -
locale init still needs a real locale file to boot into, even with the
asset-moving/archiving done separately.

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

test/save/savetestfixture.h

@@ -35,21 +35,3 @@ int saveTestFixtureTeardown(void **state);
  * filesystem permission tricks. Must be called after saveTestFixtureSetup.
  */
 void saveTestFixtureBlockSaveDirectory(void);
-
-/**
- * Stubs the JSON asset loader's sync callback to hand back a small
- * in-memory party.jsonc-shaped doc instead of doing a real file/zip read -
- * saveInit()/saveSlotInit() seed the party roster via partyInit(), which
- * loads party.jsonc through the real asset system, and there's no real
- * asset bundle available under the test harness. No thread, no real I/O.
- * Resets ASSET to a fresh, otherwise-empty state each call, so it's safe
- * to call once per test. Called automatically by saveTestFixtureSetup.
- */
-void saveTestFixtureStubPartyAsset(void);
-
-/**
- * Frees the in-memory doc created by saveTestFixtureStubPartyAsset. Safe
- * to call even if that function was never called. Called automatically
- * by saveTestFixtureTeardown.
- */
-void saveTestFixtureUnstubPartyAsset(void);