move_cards only checked that the *bottom* card of a moved stack landed
legally on the destination — the cards above the bottom went through
unverified. A player could lift an arbitrary selection from one column
and drop it on another whenever the bottom happened to match, even if
the upper cards didn't form a descending alternating-colour sequence.
Adds is_valid_tableau_sequence(&[Card]) -> bool to rules.rs (4 lines)
and one call site in move_cards's tableau-destination branch. One
focused test covering single-card / valid-run / same-colour /
rank-gap cases.
Reported by Quat: "stack 4 onto stack 2" was accepted when illegal.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
funman300
f1aeb24157