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/CMakeLists.txt

@@ -7,8 +7,6 @@ include(dusktest)
 
 # Tests
 dusktest(test_savesettings.c)
-dusktest(test_saveslot.c)
-target_sources(test_saveslot PRIVATE savetestfixture.c)
 
 dusktest(test_savedevicelinux.c)
 target_sources(test_savedevicelinux PRIVATE savetestfixture.c)