A model specification is versioned plain data. Compilation validates the alphabet, brackets, weights, callback IDs, and production index. A word is a sequence of immutable parameterized symbols. A derivation rewrites all symbols in parallel; an identity production is implicit when nothing matches.
A turtle interprets the final word but is not part of rewriting. It emits a branch graph and polygon paths. Geometry builders may turn that topology into meshes, and optional renderers may turn results into Instances.
Determinism includes the model, seed, global parameters, absolute time, and limits. Changing a limit may intentionally change the deterministic partial result. The seed is not a secret.