Design proposal for an image_type discriminator on Docker templates plus
a compute-side profile registry: vendor parameters graduate from
environment markers into schema-gated fields, generic-feature
applicability becomes declared capability data, existing markers stay as
a compatibility fallback. Explicitly not a new node type.
Four profiles already exist to shape it (iol-runner, XRd, SR Linux
prototype, FRR appliance); implementation follows the current PR series.
