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-docker.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+docker build -t dusk-linux -f docker/linux/Dockerfile .
+docker run \
+  --rm \
+  -v "${GITHUB_WORKSPACE}:/workdir" \
+  dusk-linux \
+  /bin/bash -c "./scripts/linux/test-linux.sh"