Render one of a style's two template images — a REAL step of style
setup, not an optional extra: a style isn't finished until both its
character and environment templates are rendered (the app shows them on
the style card). Asset reference images render against them (characters →
character template; environments and objects → environment template), and
segment renders fall back on them when a shot has no asset reference — so
finish BOTH before generate_asset_reference. Run once per template_type
("character" | "environment") for every new style; skip types the style
already has (get_style's `templates`). A template already exists is a hard
stop here — the call refuses unless replace=True, because overwriting one
silently re-anchors every future render. Async — await_jobs(style_id=...),
then get_style.
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