Implements the measurable test from COMPREHENSION-READER-AUDIT.md's Q5,
generalized to the deduction and curriculum composers: a deterministic
refusal histogram split mechanism/coverage/engine_refused, plus an
admissions-per-batch counter for measuring a future lexicon or curriculum
expansion's effect as a before/after delta instead of asserting it.
Found and fixed a real classification bug while building it: an argument
successfully read by a band but declined by the ROBDD engine for
inconsistent premises was being misattributed to whichever reader band
happened to run last in the fallback cascade. That case is neither a
shape gap (mechanism) nor a vocabulary gap (coverage), so it gets its own
"engine_refused" bucket rather than being folded into either.
Baseline measurement: deduction refusals are 100% mechanism-class today
(the bands read a closed connective grammar, not open vocabulary) —
curriculum refusals are a real mechanism/coverage mix, confirming the
volume ceiling S6 quantifies independently from the ratified-chain side.
