Speculative decoder paired-trace lab

Local-only synthetic analyzer. It is not an inference engine or EAGLE simulator, never extrapolates paper speedups, and never names a universal winner.

Comparison contract: every supplied method, including the optional no-speculation control, must match the n-gram baseline on shared manifest keys and on every request ID, cohort, output position, and output digest. One mismatch makes the receipt ineligible and excludes that method from aggregates, cohort views, control display, and decision effects.