rugnux: dataset Wilson B-factor estimate + robust per-image Wilson B

Add a dataset-wide isotropic Wilson B-factor estimate, the analogue of XDS's
"WILSON LINE ... B=" which we did not export. CalcGlobalWilsonB fits ln<I> vs
1/d^2 over the merged reflections (B = -2*slope), skipping the low-resolution
non-linear region (d > 4 A) and shells past the signal limit (<I/sigma> < 1)
so the estimate is insensitive to how far the merged data were carried. It is
diagnostic only - not fed back into scaling - and is written to the mmCIF
(_reflns.B_iso_Wilson_estimate), the printed merge statistics, and the log.

Also harden the per-image Wilson B (CalcWilsonBFactor): accept the fit only
when it is well-correlated and physically plausible (0 < B < 200 A^2), else
leave b_factor unset. A bad frame (an indexing glitch, too few reflections)
otherwise produced a wildly steep Wilson line and a B of several hundred A^2
that polluted the per-image plot; NaN is preferable to garbage.

Diagnostic-only: the merged intensities and every merge statistic are
byte-identical (verified baseline vs modified on the rotation battery).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

image_analysis/WriteReflections.cpp

@@ -144,6 +144,10 @@ void WriteMmcifReflections(const std::vector<MergedReflection> &reflections,
         out << "_reflns.pdbx_Rrim_I_all           " << Fmt(ov.r_meas, 4) << "\n";
         out << "_reflns.pdbx_CC_half              " << Fmt(ov.cc_half, 4) << "\n";
         out << "_reflns.jfjoch_diffrn_ISa             " << CifStr(isa) << "   # asymptotic I/sigma (Diederichs)\n";
+        // Dataset-wide isotropic Wilson B-factor estimate (standard PDBx item), analogous to XDS's
+        // "WILSON LINE ... B=". Emitted only when the log-linear fit succeeded.
+        if (std::isfinite(statistics.wilson_b) && statistics.wilson_b > 0.0)
+            out << "_reflns.B_iso_Wilson_estimate     " << Fmt(statistics.wilson_b, 2) << "\n";
         // Twinning indicators (no standard mmCIF item; same jfjoch local prefix as ISa above).
         if (twinning.l_test_pairs > 0) {
             out << "_reflns.jfjoch_L_test_mean_abs_L      " << Fmt(twinning.mean_abs_l, 3)