Exp 0 pre-registration freeze

Freezes Experiment 0's quantization recipe, metrics, and numeric go/no-go thresholds BEFORE any quantized evaluation. Platform validated: FP16 AR control reproduces BD3-LM Table 5 zero-shot perplexities within 1.6% (exact likelihood); dLLM bound consistent with published values. Eval protocol: bd3lms @ 1c3e8f4, insert_valid_eos=False. GitHub's release timestamp is the server-side pre-registration record; the attached exp0.yaml is the frozen config at this tag.