v1.5.1 — instruments from the-angular-observer: behavioral agreement, rank certificates, (A2) probe

Theory-to-practice round: the companion paper's results land as instruments, following the house pattern of turning each incident into an instrument.

• behavioral_agreement — a decision-level quantization-quality metric: flip_rate (symmetric McNemar — regressions and recoveries), prediction-level agreement, and a noise_floor control so drift is reported as excess-over-floor with a z-score. The de-confounded companion experiment shows that at matched bits V/O are functionally 2.3–6× more quantization-sensitive than Q/K across three architectures.
• rank_certificate — distribution-free rank-agreement floors for compressed retrieval: measured distortion κ, one-pass concentration μ̂, guaranteed Kendall τ ≥ 1−2μ̂ / Spearman ≥ 1−3μ̂, and max_certifiable_kappa as the per-corpus vacuity threshold.
• a2_probe — the (A2) consumer-metric probe: calibration-time quantizer-family selection against the declared consumer; reproduces the v1.2.0 KV-keys catastrophe as a unit test.
• QualityMonitor streaming (A2) tangential-fraction statistic + check_radial_drift(); two pre-registered CPU theory experiments.

(1.5.0 was an incomplete release from a partial tree and is superseded; 1.5.1 is the first release carrying the full tree.)

Full details: CHANGELOG.md.