Move tiny_gltf.h/.cc, tinygltf_json.h, json.hpp, stb_image*, loader_example.cc,
examples/, wasm/, experimental/, legacy tests and fuzzers under attic/.
TinyGLTF v3 C (tiny_gltf_v3.h/.c + tinygltf_json_c.h) is now the mainline:
- CMakeLists.txt: v3 C tests only, installs only v3 files
- tests/Makefile and Makefile: build/run the v3 C testers
- test_runner.py: verifies v3 C tester output only (no v1 ground truth)
- CI workflows: build and test v3 C only (GCC/Clang/MSVC/Meson/sanitizers)
- README: v3 C quick start, testing, licenses; legacy moved to attic/
