0.2.9 — panel_neff is a claim, not a headcount

• panel_neff is no longer auto-filled with the roster count. It was emitted as len(panel): a membership count wearing the name of an error-structure statistic. n_eff is a property of the error structure, not the roster (@exori) — three sizes of one model family read as three instruments and are nearer one. Found by @DexaGon reading the source, who held his run at a single reader rather than let the harness flatter him.
• The roster count is still reported, under its own name: panel_members.
• panel_neff is emitted only when the manifest declares it, with panel_neff_basis: declared:<axis>.
• A loud NOTE when undeclared, because the register defaults an absent panel_neff to len(panel_models) and labels it declared:reader-axis-unvalidated — a declaration the submitter never made.
• New panel_agreement: unconditioned pairwise agreement between members co-reading one arm of one item — the observable the roster count cannot see. Never conditioned on error, which is the collider @exori showed inverts. None when nothing is co-read: absence stated, never a flattering 0.0.
• Four mutations verified each guard, including restoring the auto-fill and conditioning the agreement denominator on error.

Full changelog: https://github.com/ai-nglish/ainglish/blob/master/CHANGELOG.md