symmetry: carry the space group as the group, not as its number

The adopted space group travelled the pipeline as a bare int and was rebuilt
downstream with find_spacegroup_by_number, which returns the reference setting.
So every setting a number cannot name was destroyed one line after it was
determined: P 1 1 2 came back as P 1 2 1, I 1 1 2 as C 1 2 1, R 3:R as R 3:H.

DatasetSettings now holds the gemmi::SpaceGroup itself, DiffractionExperiment
exposes it as GetGemmiSpaceGroup() / GetSpaceGroupOrP1(), and everything that
used to take an int - HKLKeyGenerator (its int constructor is gone, so the
compiler finds the callers), the merge, the R-free flags, French-Wilson, the
reindexing ambiguity, the completeness enumeration, the MTZ and mmCIF exports,
the model validation - takes the group. -S keeps the setting the symbol names
rather than reducing it to a number.

The end message carries both spellings and a reader prefers the name, since
only the name keeps the setting while the number is what a reader written
before the name understands. It carries them over CBOR too: the determined
group was never serialised at all, so a group rugnux chose reached the master
file only when the same process wrote it, and an online writer fell back to
whatever the user had supplied at the start. Both keys are optional additions,
so an older reader skips them and a newer one reads an older sender.

On disk the master's /entry/sample/space_group carries the extended
Hermann-Mauguin name and is what the reader takes the group from, so a setting
survives a _process.h5 and the --mode scale that re-reads it; the number stays
beside it and is the fallback for files written before. Every one of the 230
reference settings the old writer could produce reads back as itself, so older
files are unaffected.

Stage A and Stage B of the search still enumerate reference settings only, so
this determines no group differently today - it is what the enumeration needs
before it can be widened.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EFEJG6WBQv8th4UJFNe53N

image_analysis/WriteReflections.cpp

@@ -101,7 +101,7 @@ std::vector<MergedOutRow> BuildMergedRows(const std::vector<MergedReflection> &r
         // Anomalous: group the two mates by their (shared) Friedel-merged ASU representative, then form
         // IMEAN / F as their inverse-variance Friedel mean. A single generator gives both the group key
         // (its hkl, identical for +hkl and -hkl) and which mate this row is (.plus).
-        const HKLKeyGenerator key_gen(false, experiment.GetSpaceGroupNumber().value_or(1));
+        const HKLKeyGenerator key_gen(false, experiment.GetSpaceGroupOrP1());
         struct AnomRow {
             int h = 0, k = 0, l = 0;
             float Ip = NAN, sIp = NAN, Im = NAN, sIm = NAN;
@@ -186,13 +186,11 @@ void WriteMmcifReflections(const std::vector<MergedReflection> &reflections,
     out << "_cell.angle_beta   " << Fmt(unitCell.beta, 2) << "\n";
     out << "_cell.angle_gamma  " << Fmt(unitCell.gamma, 2) << "\n";
 
-    auto *sg = gemmi::find_spacegroup_by_number(experiment.GetSpaceGroupNumber().value_or(1));
-    if (sg == nullptr)
-        throw std::runtime_error("WriteMmcifReflections: invalid space group number");
+    const gemmi::SpaceGroup &sg = experiment.GetSpaceGroupOrP1();
 
     // ---------- _symmetry ----------
-    out << "_symmetry.space_group_name_H-M  " << CifStr(sg->hm) << "\n";
-    out << "_symmetry.Int_Tables_number      " << sg->number << "\n";
+    out << "_symmetry.space_group_name_H-M  " << CifStr(sg.hm) << "\n";
+    out << "_symmetry.Int_Tables_number      " << sg.number << "\n";
     out << "#\n";
 
     // ---------- _diffrn_source / _diffrn_detector ----------
@@ -439,8 +437,7 @@ void WriteMtzReflections(const std::vector<MergedReflection> &reflections,
     gemmi::Mtz mtz;
 
     // Optional but recommended metadata
-    mtz.spacegroup = gemmi::find_spacegroup_by_number(
-        experiment.GetSpaceGroupNumber().value_or(1));
+    mtz.spacegroup = &experiment.GetSpaceGroupOrP1();
     mtz.set_cell_for_all(unitCell);
 
     // Producing-software provenance in the MTZ header (title + HISTORY, the CCP4 convention).
@@ -698,7 +695,7 @@ void WriteUnmergedMtzReflections(const std::vector<IntegrationOutcome> &outcomes
                                  bool sum_partials,
                                  const std::string &filename) {
     gemmi::Mtz mtz;
-    mtz.spacegroup = gemmi::find_spacegroup_by_number(experiment.GetSpaceGroupNumber().value_or(1));
+    mtz.spacegroup = &experiment.GetSpaceGroupOrP1();
     mtz.set_cell_for_all(unitCell);
     mtz.title = "Rugnux unmerged observations";
     mtz.history.push_back("From Rugnux " + jfjoch_version() + ", data reduction");