nx_decode_kind.nx

nx_decode_kind.nx -- the RV64IM instruction-class DECODER as a GATE-NETWORK (the next module after the ALU, toward a bootable gate-level CPU). Mirrors the behavioral nx_rv64im_decode_kind op-for-op: extract opcode/funct3/funct7, then a CONST + EQ-match + cascade-MUX over the opcode (same shape as the ALU op- select), with the M-extension sub-decode for the R-type opcodes 0x33/0x3b. Output = NX_RV64IM_OP_* (the op class the executor routes to the ALU/loads/ branches). Verified EXHAUSTIVELY: decode_kind depends only on the 17 bits {opcode[6:0], funct3[14:12], funct7[31:25]}, so all 2^17 combos are checkable. license_tier: ORIGINAL