docs: distinguish conservative screening from virial milestone

experiments/check_plateau_physics_README.md

@@ -14,6 +14,12 @@ It reads `metadata.txt` and `final_physical.csv` and prints standard JSON. Exit
 
 The eight checks comprise energy and force virials with both P(h) and P(rho), each at quadrature orders 14 and 18. An undefined check is distinguished from a finite check exceeding the threshold. Pressure validity, density/enthalpy signs, negative-density budgets, mass/J errors, and local EOS/Bernoulli measurements are reported separately. Nonfinite numbers become JSON null with a retained finite flag and NaN/infinity classification.
 
+This joint solver-and-eight-virial screen is deliberately stricter than the primary
+physical milestone of virial balance within 1e-5. The n=1.5 endpoints meet that
+milestone using P(h); an undefined P(rho) caused by small signed DG undershoot
+does not by itself demonstrate a physical virial failure. The JSON keeps those
+individual outcomes separate from its conservative aggregate result.
+
 Passing this gate is **not full physical validation**. No new tolerance is assigned to mass/J or local errors. Sampled signs and extrema are not certified pointwise bounds. The script trusts the diagnostic's recorded convergence flag and physical measurements; it does not reconstruct a model, validate provenance, or independently recompute integrals. It does not inspect or modify production code.
 
 Verification on the September 16 campaign: