No test covers the empty-CAS guard in _predict_admixed

[dvitale199]

The guard at
genotools/ancestry/model.py:687

if len(cas_train) > 0:

is what stops Birch.fit receiving a (0, 3) array when the reference panel has
no CAS samples. In 1.x that call was unguarded and raised
ValueError: Found array with 0 sample(s) — a real user hit it with an
HGDP + 1KG panel (#202), and it would have hit any panel without CAS.

No test covers it. Nothing under tests/ exercises _predict_admixed with a
panel lacking CAS, so the fix is real but ungated: deleting the if would not
turn the suite red.

Per CLAUDE.md, the test should be revert-checked — break the guard at the
production call site, confirm the new test fails, restore — rather than driving a
helper that happens to sit nearby.

Related: the hardcoded "CAS" label itself is #293.