Rename `llm_model` → `model` and `llm_provider` → `provider` in stylesheet
properties, accessor methods, and all DOT/doc references. Add
ProviderInferenceTransform that automatically infers provider from the model
catalog, eliminating redundant provider declarations in stylesheets.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Stores DOT source in ManagedPipeline and pipes it through `dot -Tsvg`
on request. Returns image/svg+xml on success, 502 if graphviz is
unavailable, 404 if pipeline not found. Resolves spec gap #1.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>