grid scan: say how the stationary spindle angle is stated, and pin it

A grid scan is a set of stills at a stationary spindle, and the angle it stood
at is what relates one grid to another taken elsewhere on the circle. Users were
finding an all-zero omega in the file and concluding the angle could not be
recorded at all.

It already can, and has since the goniometer and the grid scan stopped being
alternatives: send the axis with step 0 and its start angle, and that angle is
written per image into the NXmx sample chain, read back by reader/, and taken by
dials.import as a set of stills. Measured on a generated 12-image grid: the
placeholder file carries omega = 0 x 12, the same file with the axis sent at
step 0 carries omega = 90 x 12, and dials.import reports "still: 1, sweep: 0"
for both. Nothing in the code needed changing, so nothing was; what was missing
was that nobody could tell, and that no test held the behaviour down.

So: the API and the HDF5 documentation now say it in as many words, and three
tests pin the three legs the value crosses - the OpenAPI request (which used to
drop the grid scan whenever an axis was present, unpinned until now), the CBOR
start message, and the file round trip.

Also corrects a claim two comments and the HDF5 page were making. NXmx can
express "no rotation" perfectly well - a sample may depend_on "." - so the
placeholder is not there for the standard's sake. It is there because dxtbx
cannot read a sample chain of translations alone: strip the rotation axis from a
grid scan master and dials.import dies in get_dxtbx_goniometer with a matmul
dimension mismatch. Recorded so nobody removes the placeholder on the strength
of the standard.

One thing the change does not fix, because it cannot: a stationary angle is
invisible to DIALS when a grid scan is present. dxtbx picks the first varying
axis as the scan axis, which is a grid translation, so the oscillation reads
(0, 0); and with exactly one rotation axis in the chain it builds a single-axis
goniometer whose fixed rotation is the identity, never consulting the angle. The
same angle IS visible when it is the only candidate (oscillation reads (90, 0))
or when a Smargon head puts a second rotation axis in the chain (the setting
rotation then carries it). The value is in the file and correct either way.

tests/DiffractionExperimentTest.cpp

@@ -8,7 +8,9 @@
 #include "../common/NetworkAddressConvert.h"
 
 #include <nlohmann/json.hpp>
+#include "Dataset_settings.h"
 #include "Dataset_settings_smargon.h"
+#include "../broker/OpenAPIConvert.h"
 
 using namespace std::literals::chrono_literals;
 
@@ -1238,3 +1240,25 @@ TEST_CASE("DiffractionExperiment_SmargonChiReadoutNoise", "[DiffractionExperimen
     REQUIRE(noisy.GetValues().size() == zero.GetValues().size());
     CHECK(noisy.GetValues().at(0) == Catch::Approx(zero.GetValues().at(0)).margin(1e-6));
 }
+
+// A grid scan and a goniometer axis are not alternatives: a grid is collected at a stationary
+// spindle, and the axis sent with step 0 is what states the angle it stood at. The request used to
+// drop the grid scan whenever an axis was present, and nothing pinned it.
+TEST_CASE("DiffractionExperiment_GridScanAtStationaryAxis", "[DiffractionExperiment]") {
+    org::openapitools::server::model::Dataset_settings model;
+    from_json(nlohmann::json::parse(R"({
+        "images_per_trigger": 12,
+        "beam_x_pxl": 512.0, "beam_y_pxl": 256.0,
+        "detector_distance_mm": 150.0, "incident_energy_keV": 12.4,
+        "grid_scan": {"n_fast": 4, "step_x_um": 20.0, "step_y_um": 20.0},
+        "goniometer": {"name": "omega", "start": 90.0, "step": 0.0, "vector": [-1, 0, 0]}
+    })"), model);
+
+    const auto settings = Convert(model);
+    REQUIRE(settings.GetGridScan().has_value());
+    CHECK(settings.GetGridScan()->GetNFast() == 4);
+    REQUIRE(settings.GetGoniometer().has_value());
+    CHECK(settings.GetGoniometer()->GetName() == "omega");
+    CHECK(!settings.GetGoniometer()->IsScanning());
+    CHECK(settings.GetGoniometer()->GetStart_deg() == Catch::Approx(90.0f));
+}