experiment: record pressure positivity and shape curvature evidence

experiments/plateau_diagnostic_README.md

@@ -193,3 +193,16 @@ between the closure and pressure enthalpy bounds and compares surface traces at
 onset and late plateau. The candidate is generated from captured normalized base
 and direction at the arithmetic midpoint of those two measured bounds; its
 derivation and hash are retained with the supplemental input.
+
+The surface trace output also records actual FE nodal h and dh values on each
+limiting element, obtained through full physical-field scatters and evaluations
+at `GetNodes()`. Shared elements are emitted once per quadrature family. This
+allows sign comparisons at arbitrary alpha without confusing coefficient signs
+with interior polynomial positivity. The union-bound column is an affine
+prediction at the boundary, not a separately prepared candidate.
+
+The completed campaign and evidence limits are recorded in
+`PRESSURE_AND_SHAPE_CAMPAIGN_2026-09-17.md`. Portable offline analyses are in
+`plateau_analysis/`; pass the retained artifact root explicitly. The Q3 follow-up
+checks a finite interior grid against the exported polynomial and does not
+certify positivity everywhere.