benchmark: extend Panel estimator and covariance coverage

[TheHiddenObserver]

Goal

Extend canonical Panel coverage beyond the currently aligned PanelOLS and RandomEffects rows.

Scope

• audit older Panel/PooledOLS evidence with incomplete backend matrices or weak scale identity;
• cover maintained pooled, fixed-effects, random-effects, first-difference, between, Fama-MacBeth, and covariance variants where applicable;
• align formulas, entity/time effects, covariance modes, scales, external references, and backend timing;
• add structured sources, parser tests, registration, generated assets, and coverage-matrix updates.

Evidence with incomplete identity remains non-canonical until rerun.

Parent: #100