space group: determine centering from absences; jfjoch_process re-indexes from scratch

Fixes wrong space-group assignment on cubic insulin (true I2_13, a=77.6).

SearchSpaceGroup: drop the lattice-centering gate (and the now-unused
lattice_centering option). The indexer often returns the conventional cell,
whose geometry hides I/F/C centering - that information lives only in the
systematic absences. Stage B now tests every centering of the point group and
confirms it from the data, so an indexer-reported 'P' no longer excludes
I2_13/I23. (I23 and I2_13 remain indistinguishable by absences and are reported
as such.)

jfjoch_process: discard any unit cell stored in the input HDF5 by default so the
cell is re-determined from scratch. A stale/wrong stored cell otherwise resolves
the indexing algorithm to FFBIDX, which trusts that cell and locks onto the wrong
lattice (Ins_I_3 went from 2.7% -> 76% indexing). A user-supplied -C cell still
applies.

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

process/JFJochProcess.cpp

@@ -481,11 +481,9 @@ ProcessResult JFJochProcess::Run(JFJochProcessObserver *observer) {
             sg_opts.merge_friedel = experiment_.GetScalingSettings().GetMergeFriedel();
             sg_opts.d_min_limit_A = experiment_.GetScalingSettings().GetHighResolutionLimit_A().value_or(0.0);
             // Constrain the search to subgroups of the lattice (metric) symmetry found by rotation
-            // indexing, and weigh centering only against the metric one.
-            if (end_msg.rotation_lattice_type.has_value()) {
+            // indexing. Centering is not constrained here - it is determined from the absences.
+            if (end_msg.rotation_lattice_type.has_value())
                 sg_opts.lattice_system = end_msg.rotation_lattice_type->crystal_system;
-                sg_opts.lattice_centering = end_msg.rotation_lattice_type->centering;
-            }
             const auto sg_search = SearchSpaceGroup(sm.merged, sg_opts);
             stats_text << SearchSpaceGroupResultToText(sg_search) << "\n\n";