Remove rpg, ui, editor and scene subfolders; split scripts by platform

Delete src/dusk/rpg, src/dusk/ui, the animation/keyframe modules, the
scene subfolders, the chunk asset loader and ./editor, and strip the
dependent save slot, scene type, engine and test code so the main
binary and tests build again. Add scene/node.c/.h skeleton and move
build scripts into scripts/<platform>/.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

test/save/savetestfixture.h

@@ -36,20 +36,3 @@ int saveTestFixtureTeardown(void **state);
  */
 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);