nx_codegen_diff_gate.nx

nx_codegen_diff_gate.nx -- DIFFERENTIAL codegen verifier: the safety gate that lets the Nishi team improve register allocation (nx_x86_regalloc) WITHOUT a g1 repeat. Compiles a corpus of self-verifying programs with TWO compilers (the pinned known-good and a candidate), links + runs both, asserts IDENTICAL runtime behaviour. A correct codegen change emits different asm but the SAME result; a g1-style high-pressure miscompile DIVERGES and is CAUGHT before promotion. Build pipeline = nx_codegen_exec (shared). Known answer: all agree -> exit 0.