Function shader_cache_dir 

pub fn shader_cache_dir() -> Option<PathBuf>

Directory the renderer writes compiled built-in shader binaries to, keyed by a hash of their compile inputs. Resolves under the writable-state dir, since a shipped install’s content root may be read-only. Distinct from cache_dir, which holds cooked asset payloads: these artifacts belong to the machine’s shader compiler, not to the build.