Magic-bitboard attack lookups for sliding pieces (rooks, bishops).

At compile time the cached magics in priv/magic_cache.bin are unrolled into per-square get_rook_attacks/2 and get_bishop_attacks/2 clauses: each masks the occupancy, multiplies by the square's magic, shifts to an index and reads the attack set from an embedded tuple table — O(1) with no runtime cache lookup. Regenerate the cache with scripts/generate_magic_cache.exs (see Echecs.Bitboard.MagicGenerator).