nx_syscall_xlate_gate.nx

nx_syscall_xlate_gate.nx -- RATCHET gate for the compiler's rv64->x86_64 syscall translation table (x86ctx_rv64_to_x86_64_syscall in nx_x86_64_ctx.nx). The process-IDENTITY family was MISSING: a const rv64 getpid(172) fell through `return num` to raw x86_64 172=iopl -> -ENOSYS (-38). That silently filled a loop-singleton pidfile with -38 and the gate FAILED OPEN (two daemons raced -- see nx_gate_loop_register X-GETPID-XLATE). nx_signal/nx_swarm_queue/nx_rv64_sim/ml_pid_alive all wanted getpid via 172 and were silently broken. This gate calls each identity syscall via its rv64/asm-generic CONST number (exercising the const-translation path I fixed) and asserts a NON-ERROR result. A -errno (e.g. -38 ENOSYS from an un-translated passthrough) is negative and fails. If ANY row is dropped from the table, this goes RED. One test per row added (172/173/174/175/176/177). license_tier: ORIGINAL expect_exit:0