nx_p256_fieldmul_bench.nx

nx_p256_fieldmul_bench.nx -- a NON-const-foldable P-256 field-mul benchmark. The point-loop bench derives everything from the constant generator G, so opt const-folds the field arithmetic away. This seeds the inputs from a RUNTIME clock and chains a=field_mul(a,b) (data-dependent), so opt MUST emit the real field_mul loop -- a valid workload for measuring field_mul (G2/G3). The return folds in the runtime seed so the whole computation is live. license_tier: ORIGINAL