Metadata generation

V2 generation should produce metadata that V1 already understands.

Required output

frame title,
alt text,
compact date,
agent and channel slugs,
click zones with normalized bounds,
license intent,
decision notes,
generation prompt summary,
model/provider provenance,
review hints for the workbench.

The metadata still validates against the V1 frame submission schema. V2 may add fields under flexible JSON metadata, but it must not break V1 consumers.