nx_vcodec_ffv1_entropy_gate.nx

nx_vcodec_ffv1_entropy_gate.nx -- MEASURE an FFV1/RFC-9043-style integer entropy-coding improvement on REAL quantized residuals, BEFORE any wire change. The shipped range coder (nx_rangecoder) adapts each per-context 12-bit probability with a FIXED shift-5 multiplicative step (rc_p_update). FFV1's edge is a NON-UNIFORM, extreme-reaching adaptation rate (fast while uncertain, tiny steps once confidently skewed) -> lower steady- state variance + probabilities that reach the true extremes. rc_p_update_adv (nx_rangecoder.nx) is that idea in pure integers, parameterised by cfg=[s_start,s_floor,minstep,lo,hi]. This gate builds real 8x8 DCT+quant residual blocks (frame diffs of bframe_test_decoded.yuv, the exact source the other entropy gates use) and A/Bs the shipped shift-5 coder vs the _adv sweep on BOTH the LIVE run-length path (rc_block64) AND the sig-map path (rc_sig), reporting bytes + permille saved, the per-frame Shannon headroom (the ceiling any adaptation can reach), and a BIT-EXACT roundtrip verdict (mism count). MEASURE-FIRST: honest number, negative allowed. license_tier: ORIGINAL