[Missing] MVC/One canonical example blocks 2 CLI reduction pipelines

[zazabap]

Summary

MinimumVertexCover → EnsembleComputation and MinimumVertexCover → MinimumHittingSet require the MVC/One (unit-weight) variant as source, but only MVC/i32 (weighted) has a canonical example in example_db. This means:

pred create --example MinimumVertexCover -o mvc.json   # creates MVC/i32
pred reduce mvc.json --to EnsembleComputation -o bundle.json  # FAILS: no path from MVC/i32

Fix

Add a canonical example for MinimumVertexCover/SimpleGraph/One in src/example_db/model_builders.rs.

Also missing: KColoring/SimpleGraph/KN has no canonical example (pred create --example KColoring fails). The K3 variant has an example and works.

Origin

EnsembleComputation and MinimumHittingSet reductions from PR #804. KColoring gap is older.

Discovery Method

Same systematic CLI round-trip test as #1002. See #1005 for the CI integration proposal.

[zazabap]

Superseded by #1006. The missing examples are a secondary concern — the 4 unsound reductions are the priority.