On macOS the Vulkan loader comes from MoltenVK. SDL's default
probe only searches system paths, so a bundled libMoltenVK.dylib
next to the executable is invisible under the hardened runtime.
Probe it explicitly via SDL_GetBasePath() before falling back to
SDL's default search. Respect SDL_VULKAN_LIBRARY overrides and
log which loader is used. Update README to reflect that no
environment variable is required for bundled builds.
Co-Authored-By: internal-model
