solve_combined_rate(CombinedRateProblem) -> int | Refusal over effective_rate = rate_a +/- rate_b.
effective_rate query returns the net (even <=0); quantity/time queries need a positive net rate
(non_positive_net_rate) and exact division (non_integer_solution). Pure integer arithmetic, no
float/Fraction (CMB v1 crosses no units). Off-serving. Graded by a new run_solver lane in
evals/combined_rate_oracle against the committed gold.
Adversarial 5-lens(+adjudicator) verification returned fix_first; both real hazards fixed:
- wrong=0 breach: negative known-slot inputs (time<0/quantity<0) bypassed the eff<=0 guard and
produced negative answers. FIXED UPSTREAM in model.py __post_init__ — rate_a/rate_b and the known
time/quantity are now positive ints, so the illegal state is unrepresentable and the solver can
never receive a negative-yielding path. Added model + gold (cmb-07d eff<0/time) + lane coverage.
- doc overclaim: the solver lane does NOT grade 'two independent paths' (both solver and the
oracle's _canonical_outcome delegate to model.effective_rate). Corrected both docstrings to name
the true anchor (committed gold + inline-computed literal tests) and added a difference-mode
inline-computed test.
R3-vac came back SOLID (separate PR). gold 18/18 (6/5/7); solver lane 6/0 + 5/0; 36 CMB tests;
router-hygiene + serving unchanged.
First rung of the combined-rate ladder, off-serving (imports no generate.derivation /
core.reliability_gate). Claims exactly: the combined-rate setup ruler is defined and
gold-valid. No reader/solver/wiring yet — no capability claim.
- generate/combined_rate_comprehension/: local RateUnit (decoupled from R3), CombinedRateProblem
(two-explicit-rates + per-query-slot guard; effective_rate = rate_a +/- rate_b; non-positive net
left to the solver, not the model).
- evals/combined_rate_oracle/: span-free signature (sum commutative / difference ordered) + a
NON-VACUOUS validator. 17 gold fixtures: 6 solved (full combine_mode x query grid), 4
solver_refuses, 7 reader_refuses (the complete refusal taxonomy + the 2x2 domain-entry grid).
Adversarial 5-lens verification returned fix_first; the validator now cross-checks every solved
gold and solver_refuses reason against the canonical arithmetic (_canonical_outcome) so a
mislabelled / arithmetically-impossible fixture is rejected (meaningful-fail per CLAUDE.md), with
dedicated tests. Added eff<0 and eff=0/time coverage fixtures; removed a dead determinism guard.
gold 17/17 valid; 25 oracle tests; R1/R2/R3 + router-hygiene + serving all unchanged.
Doc: docs/analysis/cmb-a-combined-rate-ruler-2026-06-08.md
