Ice: detect it on two channels, and only handle it when it is there

The per-image ice score was read off the PLAIN azimuthal profile. That profile is a
per-ring mean, so a few strong Bragg reflections landing in a ring's q bin lift it
exactly as ice would. Measured over 37 rotation crystals, that did not merely add
noise - it INVERTED the metric: the two highest-scoring crystals had no ice at all
(4.23 and 4.06), while a clean control read 1.57. A decoy null - the identical
statistic evaluated at q positions where hexagonal ice cannot be - reaches 1.51 at its
99th percentile and 2.70 at its maximum, so that metric cannot support any absolute
threshold whatsoever.

The adaptive spot finder already computes the right input for its own threshold: a
sigma-clipped per-resolution-ring background, in the same bins. A powder ring is
azimuthally smooth and survives the clip; Bragg peaks do not. On the clipped profile
the clean population tightens to 1.00-1.22 and the crystals with confirmed ice sit at
2.08-2.37, against a decoy null that never exceeds 1.29.

That channel is blind to one thing: ice in large crystallites diffracts as DISCRETE
spots and leaves the radial profile flat. So a second channel counts found spots on the
rings against the same q width of ice-free flanks beside them. The two barely overlap -
the smooth-ice crystals read 2.1-2.4 / ~1.0 and the textured ones ~1.1 / 3.8-17.6,
while a clean crystal reads 1.04 on both.

Both are then used as a GATE (--ice-min-score 1.5, --ice-min-spot-ratio 2.0, both
calibrated on the battery, 0 disables): the eleven fixed hexagonal bands cover 16-26 %
of the unique reflections at typical resolutions whether or not the crystal has ice, so
flagging, the exclusion from the scale fit and the merge-time CC1/2 ring mask are now
all skipped when neither channel sees any. The gate is applied in the full pipeline and
in --scale, which reads the stored per-image values back out of the _process.h5.

Also fixes the merge-time mask's control: the shoulder now excludes reflections that
are themselves on an ice ring. The rings are not evenly spaced - 1.947/1.916/1.882 A
sit 0.05-0.06 apart in q - so for those three the [w,3w) shoulder landed squarely on
the neighbours and the test compared ice against ice. Measured, that is the only thing
this changes: it removes firings on those three rings and leaves every other firing's
CC pair identical to three decimals.

And the online ice half-width, which was 0.02 in the API against 0.03 offline, so the
same data got a narrower band online than the measured ~0.06 ring FWHM justifies.

Battery (37 rotation crystals, against the previous behaviour): space groups 34/37 in
both and NO crystal's space group changes; 6 crystals gain unique reflections, 1 loses.
Best of them gains 7082 unique reflections with R_meas 16.0 -> 14.3, CC1/2 95.9 -> 97.3
and ISa 13.7 -> 19.0; another goes R_meas 54.9 -> 42.9, CC1/2 84.0 -> 90.4, ISa
3.9 -> 5.5; a third reaches CC1/2 99.4 from 95.7 at an unchanged reflection count. The
one crystal that loses reflections improves on both R_meas and CC1/2.

Not done here: the ScanResult/API/plot-type/frontend/viewer layers for the new
spot_count_ice_control (they need the OpenAPI regeneration). Message, CBOR, HDF5
write/read and the receiver plots are.

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

common/ScanResultGenerator.cpp

@@ -56,6 +56,7 @@ void ScanResultGenerator::Add(const DataMessage &message) {
         v[image_number].max_viable_pixel = message.max_viable_pixel_value;
         v[image_number].sat_pixels = message.saturated_pixel_count;
         v[image_number].spot_count_ice = message.spot_count_ice_rings;
+        v[image_number].spot_count_ice_control = message.spot_count_ice_control;
         v[image_number].spot_count_low_res = message.spot_count_low_res;
         v[image_number].spot_count_indexed = message.spot_count_indexed;
         v[image_number].res = message.resolution_estimate;
@@ -93,6 +94,7 @@ void ScanResultGenerator::FillEndMessage(EndMessage &message) const {
     message.data_collection_efficiency.resize(n);
     message.spot_count.resize(n);
     message.spot_count_ice_ring.resize(n);
+    message.spot_count_ice_control.resize(n);
     message.spot_count_low_res.resize(n);
     message.spot_count_indexed.resize(n);
     message.image_indexed.resize(n);
@@ -124,6 +126,7 @@ void ScanResultGenerator::FillEndMessage(EndMessage &message) const {
         message.data_collection_efficiency[number] = e.collection_efficiency;
         message.spot_count[number] = static_cast<int32_t>(value_or_zero(e.spot_count));
         message.spot_count_ice_ring[number] = static_cast<int32_t>(value_or_zero(e.spot_count_ice));
+        message.spot_count_ice_control[number] = e.spot_count_ice_control.value_or(NAN);
         message.spot_count_low_res[number] = static_cast<int32_t>(value_or_zero(e.spot_count_low_res));
         message.spot_count_indexed[number] = static_cast<int32_t>(value_or_zero(e.spot_count_indexed));
         message.image_indexed[number] = static_cast<uint8_t>(e.indexing_solution.value_or(0));