Add session-mode code generation that produces a reusable WorkflowSession class
instead of a simple oneshot script. The session mode generates code with an
in-class run/close lifecycle for warm ComfyUI reuse.
Also refactors render.py to extract shared code sections into common methods,
reducing duplication between oneshot and session renderers.
Adds WorkflowSessionRuntime with cleanup policies (per_run, session, manual)
and reset_every_n_runs support, plus 72 new tests covering session rendering,
runtime session, and session export pipeline.
