fix(atomic): validate en passant with variant rules

[0d00ciallo0721]

Fixes #1214

Summary

AtomicBoard inherits Board.generate_legal_ep(), which only checks whether the capturing king remains in check. Atomic chess has two additional cases: capturing the opponent's king makes the move legal, while exploding the capturing king makes it illegal. Use AtomicBoard.is_legal() for en-passant generation, matching generate_legal_moves().

The regression coverage exercises both cases through generate_legal_moves(), generate_legal_ep(), generate_legal_captures(), has_legal_en_passant(), FEN, and EPD.

Verification

• python3 test.py AtomicTestCase.test_atomic_en_passant --verbose — passed.
• python3 test.py — 296 tests passed, 22 skipped because optional chess engines/tablebases are unavailable locally.
• git diff --check — passed.
• The README doctest produced 66 passes; three engine examples could not run because Stockfish is not installed in the local environment. The repository CI setup installs Stockfish before running that doctest.

AI assistance was used for issue analysis, the implementation, and regression coverage.