WriteReflections: write merging statistics + ISa to the mmCIF output

The .cif (--scaling-output cif) now carries the per-shell and overall merging
statistics in the standard _reflns / _reflns_shell categories (resolution, redundancy,
completeness, <I/sigma>, R-rim/R-meas, CC1/2) plus the Diederichs asymptotic I/sigma
(ISa) as a free-text _reflns.pdbx_diffrn_ISa item (no standard CIF tag exists). The
MergeStatistics and the ISa string are threaded through WriteReflections to the mmCIF
writer; jfjoch_process and jfjoch_scale pass them. Values match the text statistics table.

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

process/JFJochProcess.cpp

@@ -671,7 +671,9 @@ ProcessResult JFJochProcess::Run(JFJochProcessObserver *observer) {
 
         if (result.consensus_cell && write_files) {
             phase("Writing reflections");
-            WriteReflections(sm.merged, *result.consensus_cell, experiment_, config_.output_prefix);
+            WriteReflections(sm.merged, *result.consensus_cell, experiment_, sm.statistics,
+                             result.error_model_isa > 0 ? fmt::format("{:.2f}", result.error_model_isa) : "?",
+                             config_.output_prefix);
             // Per-image scaling table (G, B-factor, mosaicity, wedge, CC) for inspection / XDS
             // comparison. The offline self-scaling result is otherwise not exposed (process.h5's
             // per-image arrays are only filled on the online per-image path). Sourced from the