rugnux: do not re-find the rotation spots just because --detect-ice-rings was named

First-pass rotation indexing reuses the spots the acquisition wrote, and does
NOT re-mark them - their ice flags come from the file. So --detect-ice-rings can
only invalidate those spots when it asks for something the file did not do.

It was flagged as a spot-finding option, which forced reuse_rotation_spots off
whenever it appeared, so naming it swapped the acquisition's spots for this
program's own and moved the first-pass lattice by itself. Measured over the
37-crystal rotation battery, passing the SEMANTICALLY NULL --detect-ice-rings=on
to files that already carry detect_ice_rings=1 changed the merged data on every
crystal, sent one crystal's ISa from 1.66 to 0.38, and lost MyoB_13 to indexing
failure outright. Both arms of any A/B on the flag therefore moved for a reason
that had nothing to do with ice, which made the flag impossible to test.

Re-find only when the requested value differs from the file's, and say so when
it happens. A file with no key at all counts as "did not mark", which is what
its stored spots show - such a dataset carries no per-spot ice flags to reuse.

Verified over the battery, with the merge mask and the radial background pinned
off so this is the only variable. --detect-ice-rings=on on the 36 keyed
crystals: all 36 log "using the spots stored in the file", the re-finding line
appears nowhere in the arm, unique counts are identical on every crystal, the
largest mean |dI|/sigma is 3.4e-5 against a repeat-run floor of 9.3e-5, and not
one of R_meas, CC1/2, CC1/2_hi, ISa, completeness, SigAno, d_min or space group
differs anywhere. MyoB_13 indexes again. The one file carrying no key reuses
under =off and re-finds under =on, as it should.

--detect-ice-rings=off still re-finds, since it does differ from those files,
and two crystals still fail to index there. That is not this change: a control
that re-finds with ice marking ON indexes both. With the marking off, ice spots
are no longer ordered last, so they consume the --max-spots budget and the
first pass collapses.

With the confound removed the flag can finally be measured, and on a comparison
whose spot source is identical on both arms it is clearly worth having - though
the win is at INDEXING rather than at the scale fit. Three crystals are saved
outright (one would otherwise collapse to P1 at CC1/2 19%, one loses half its
completeness and its screw axis, one loses its F-centred cubic lattice), two
more only index with it on, and the remaining eight gate-fired crystals differ
by well under 1% in R_meas and CC1/2 in both directions.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

rugnux/rugnux_cli.cpp

@@ -849,7 +849,8 @@ static int RunRugnux(int argc, char **argv) {
             scale_fulls_arg = false;
             break;
         case OPT_DETECT_ICE_RINGS:
-            spot_finding_options_given = true;
+            // Deliberately NOT a spot-finding option for the purpose of re-finding the first-pass
+            // rotation spots - see where reuse_rotation_spots is decided below.
             if (optarg == nullptr || strcmp(optarg, "on") == 0)
                 detect_ice_rings = true;
             else if (strcmp(optarg, "off") == 0)
@@ -1591,6 +1592,27 @@ static int RunRugnux(int argc, char **argv) {
     else if (!dataset->file_detect_ice_rings.has_value())
         experiment.DetectIceRings(rotation_indexing);
 
+    // First-pass rotation indexing reuses the spots stored in the file, and does NOT re-mark them -
+    // their ice flags are the ones the acquisition wrote. So --detect-ice-rings can only invalidate
+    // them when it asks for something the file did not do; matching the file is a no-op for that pass.
+    // Re-finding whenever the flag is merely present would swap the acquisition's spots for this
+    // program's own, which moves the first-pass lattice by itself: measured over the rotation battery,
+    // passing the semantically null --detect-ice-rings=on to files that already carry it changed the
+    // merged data on every crystal and lost one to indexing failure. That also made the flag impossible
+    // to test, since both arms of an A/B moved for a reason unrelated to ice.
+    // A file with no key at all is treated as "did not mark", which is what its stored spots show:
+    // such a dataset carries no per-spot ice flags to reuse. That is a statement about the SPOTS, not
+    // about the setting - which is why it stays value_or(false) even though a keyless rotation file
+    // now defaults to detecting ice.
+    if (detect_ice_rings.has_value()
+        && detect_ice_rings.value() != dataset->file_detect_ice_rings.value_or(false)) {
+        if (reuse_rotation_spots)
+            logger.Info("--detect-ice-rings={} differs from the spots stored in the file: re-finding "
+                        "them for first-pass rotation indexing too",
+                        detect_ice_rings.value() ? "on" : "off");
+        reuse_rotation_spots = false;
+    }
+
     // Scale-fulls refits the per-frame scale on the rotation combined fulls; on by default for rotation
     // data (where it lifts ISa substantially) and off for stills. --no-scale-fulls overrides.
     const bool scale_fulls = scale_fulls_arg.value_or(rotation_indexing);