rugnux: name the unmerged export from the prefix, and drop centring absences

--export-unmerged and --export-unmerged-partials no longer take a filename;
they write <prefix>_unmerged.mtz and <prefix>_unmerged_partials.mtz, as
--write-process-h5 already does for <prefix>_process.h5.

Lattice-centring absences are no longer written. They are integrated on
purpose - prediction runs in P so the space-group search can confirm or
disprove the centring - but in an exported file they cost the reader the
answer: pointless reads the interstitial nodes, takes the lattice for
primitive, and demotes I23 to P2_1 3, while aimless roughly halves its
<I/sigma> merging them. They were also only present when the indexer had
settled on the conventional centred cell, so the file's content depended on
which of two equivalent cells it picked.

Screw and glide absences are still written. They are the evidence a space
group is chosen on, so a program reading this file can check that choice
independently; deleting them turns pointless's test into an assumption
("we cannot determine if the reflections are truly systematically absent").
Deleting them changes no merging statistic. XDS and DIALS draw the line in
the same place - XDS ships screw absences and never measures a centring one,
and DIALS's absence machinery has only screw-axis classes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CHMmeM1d489zvNFT7ZMN2P

image_analysis/WriteReflections.cpp

@@ -5,6 +5,7 @@
 #include "scale_merge/Merge.h"
 #include "scale_merge/HKLKey.h"
 #include "scale_merge/TwinningAnalysis.h"
+#include "bragg_integration/SystematicAbsence.h"
 #include "../common/ParallelFor.h"
 
 #include <algorithm>
@@ -692,7 +693,18 @@ void WriteUnmergedMtzReflections(const std::vector<IntegrationOutcome> &outcomes
     // it, and every program this file is for wants to handle it its own way.
     gemmi::UnmergedHklMover hkl_mover(mtz.spacegroup);
     std::set<int> batch_numbers;
+
+    // Lattice-centring absences are integrated on purpose - prediction runs in P so the space-group
+    // search can confirm or disprove the centring - but they are not written here. POINTLESS reads
+    // the interstitial nodes, takes the lattice for primitive and demotes the space group, and they
+    // roughly halve AIMLESS's <I/sigma>. Screw and glide absences ARE written: they are the evidence
+    // the space group was chosen on, so a program reading this file can check that choice, and
+    // removing them turns its test into an assumption. XDS and DIALS draw the line in the same place.
+    const char centering = mtz.spacegroup ? mtz.spacegroup->hm[0] : 'P';
+
     const auto add_row = [&](const Reflection &r) {
+        if (systematic_absence(r.h, r.k, r.l, centering))
+            return;
         std::array<int, 3> hkl{r.h, r.k, r.l};
         const int isym = hkl_mover.move_to_asu(hkl);
         // A summed full's image_number is its rocking-curve centroid, so this is the batch the