Answer "if I change this stratum, what happens?" so nobody has to recall a design doc.

A flavor rides a new base FOR FREE when its own layers are pure content (source, config, static binaries). It must be REBUILT when those layers contain something compiled against the base -- a venv, a native extension -- because the interpreter ABI and site-packages path move with it. That is exactly the MINOR/MAJOR distinction, made queryable.

abi_coupled is DECLARED at ingest rather than inferred: a COPY --from of a venv is indistinguishable from any other COPY in the layer history, so guessing would be worse than asking.