One family entry covers several published variants whose weights differ
(flux.1: schnell, dev, Krea-dev), but prequant resolution was keyed on
(family, scheme) alone, so only the default base could ever be served: the
loader's baked base_model_id validation correctly refused the schnell
checkpoint for dev and Krea-dev bases and every such load paid the dense
download plus on-the-fly quantise.
Add an optional prequant_variant_repos table on DiffusionFamily as
(base_repo, scheme, repo_id) triples and thread the resolved base repo
through resolve_prequant_source / usable_prequant_source and their three
call sites (load fast path, memory-plan probe, auto-policy candidate). A
base without its own entry keeps returning the family default, preserving
the existing refuse-then-dense behavior exactly.
Wire the flux.1 variants: the gate-validated unsloth/FLUX.1-dev-FP8
checkpoints (built in the earlier campaign but never reachable) and the
new unsloth/FLUX.1-Krea-dev-FP8.
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