conjugate_gaussian conformance fixture

Suite path: conformance/bayesian/conjugate_gaussian

Clean-room noiseless linear design y = 1 + 2x with known tiny residual variance. Expected posterior MAP / mean recovers OLS coefficients. Exercised by antecedent-prob unit/integration tests and gate.

Expected summary

Top-level keys: backend, notes, prior, tolerance, tolerance_class, true_coefficients (6 fields).