Implement Spell Chess variant with spell gating (Closes #736)

[Belzedar94]

Summary

• implement the Spell Chess variant, including freeze and jump potion metadata, variant flags, and king-capture victory conditions so the engine recognises the new ruleset
• extend position state, move generation, and hashing so spell zones, jump targets, and per-side cooldowns are tracked, consumed from pockets, and remain unique in the transposition table
• expose the variant to frontends and add regression coverage that exercises potion parsing plus freeze and jump gating behaviour

Testing

• make -j2 ARCH=x86-64 build
• python3 test.py (fails: ModuleNotFoundError: No module named 'pyffish')
• cd src && ./stockfish check ../src/variants.ini

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https://chatgpt.com/codex/tasks/task_e_68ccefff4f3083228b0a446a83e10926