nx_rv64_mulh_gate.nx

nx_rv64_mulh_gate.nx -- GATE: the HIGH-MULTIPLY half of the M extension (MULH/MULHU/MULHSU) -- the 64-bit high word of a 64x64->128 product, in signed / unsigned / signed*unsigned forms. Completes RV64IM (was: unimpl->halt). The interpreter computes the 128-bit high via Hacker's Delight 32-bit-half decomposition (NishiLang is 64-bit only, and its '>>' is arithmetic -> every carry shift is masked); the JIT uses x86 one-operand imul/mul (high in rdx). MULHSU has no x86 form -> fail-safe tiers to the interpreter. T1 MULHU (unsigned high): (2^64-1)^2, (2^32)^2, and the case that DIFFERS from signed. T2 MULH (signed high): the sign corrections -- (-1)*(-1) high=0, (-1)*2 high=-1, positive overflow. T3 MULHSU (signed*unsigned high): differs from BOTH MULH and MULHU on the same operands. T4 JIT MULH/MULHU == interpreter (imul/mul high-word extraction is correct). T5 MULHSU fail-safe TIERS: the JIT declines (-1), the interpreter computes it. expect_exit: 0