Expand tactical and endgame regression tests

[binarylab2022-del]

Objective

Increase correctness coverage for tactical positions, special rules, and
representative endgames.

Suggested coverage

• checks, pins, discovered attacks, and double checks;
• castling legality;
• en passant king-safety edge cases;
• underpromotions;
• repetition and fifty-move handling;
• insufficient-material positions;
• mate-distance behavior;
• make/unmake and Zobrist-hash consistency.

Acceptance criteria

• each regression position has a documented expected result;
• tests run automatically in CI;
• failures identify the position and expected behavior;
• no existing test is weakened or removed without justification.