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]>

scripts/linux/test-linux.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+set -e
+rm -rf build-tests
+cmake -S . -B build-tests -DDUSK_BUILD_TESTS=ON -DDUSK_TARGET_SYSTEM=linux
+cmake --build build-tests -- -j$(nproc)
+ctest --output-on-failure --test-dir build-tests