Expand Rust Core Unit Test Coverage

[smly]

Expand unit tests across riichienv-core modules to improve edge-case coverage.

• agari.rs: Add edge cases for special patterns (Kokushi Musou, Chiitoitsu)
• hand_evaluator.rs / hand_evaluator_3p.rs: Add tests for compound Yaku and Double Yakuman
• yaku_checker.rs: Exhaustive positive/negative tests for all Yaku
• score.rs: Combinatorial tests for Honba, Pao, Tsumo/Ron variations
• legal_actions.rs (4P/3P): Edge cases for Kan restrictions, Kuikae, post-Riichi constraints
• event_handler.rs (4P/3P): Event ordering verification tests