nx_js_jit_gate.nx

nx_js_jit_gate.nx -- P5 BASELINE JIT: prove it (A) is CORRECT vs the VM and the tree-walker on the templatable subset, (B) DECLINES cleanly (jit_compile==0 -> VM) on everything outside the subset, and (C) MEASURES the native speedup + class band on the SAME entry point (js_jit_disable A/B). The 108/108 parity gate already proves JIT==tree end-to-end; this adds the three-way A/B/tree agreement, the decline contract, and the ladder placement. expect_exit: 0 license_tier: ORIGINAL