nx_vcodec_mrbits_gate.nx

nx_vcodec_mrbits_gate.nx -- the REALIZED-BITS test of multi-reference prediction (does the vcv-7 build pay for itself?). The earlier nx_vcodec_multiref_gate measured residual SAD (an upper bound); this measures the ACTUAL CODED BITS: per 16x16 inter MB, ME on each reference -> real DCT+RDOQ residual entropy cost (ve_cost, the range coder's monotone proxy) + MV cost. Single-ref always uses N-1; multi-ref picks the cheaper of {N-1, N-2} and PAYS the 1-bit/MB ref-index. If multi-ref total bits < single-ref AFTER the overhead, the gain is REAL and the vcv-7 build is justified; if not, multi-ref loses to its own signaling and we DON'T build it (measure-first, no half-build). GREEN = reports the realized delta + it's positive. ORIGINAL.