Indexing: a coplanar cell leaves the frame unindexed instead of throwing

The online broker cancelled a collection with "Crystal lattice is coplanar and has no reciprocal
cell". The candidate filter in PostIndexingRefinement (used by FFT, FFTW and ffbidx) tests lengths,
angles and spot count only; three rows 120 deg apart in one plane pass all of them, and spots along
the plane normal index as (0,0,0). Such a candidate - from ffbidx, or one the least-squares
refinement walked flat - went on to LatticeSearch and XtalOptimizer, which refuses it and leaves it
in place, and then reached Astar() in AnalyzeIndexing/prediction, which throws; the receiver turns
that into a cancelled acquisition.

- Refine() rejects a candidate below MIN_BASIS_VOLUME_FRACTION, like any other bad candidate.
- XtalOptimizerInternal builds the refined lattice first and counts a coplanar result as a failed
  refinement, writing nothing back (the residual's volume clamp lets a solve end there and report
  success).

Test: PostIndexingRefinement_CoplanarCandidateIsRejected threw the exact message before the fix.
rugnux p.mtz unchanged on the three in-house reference sets at --prepass-fraction 1.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SVmAWnzCmRKAXVUCdc4iNi

tests/IndexingUnitTest.cpp

@@ -785,3 +785,43 @@ TEST_CASE("FitSupercellProbe", "[SupercellProbe]") {
     // Too few reflections to fit a line: nothing.
     CHECK(FitSupercellProbe(SupercellProbeClass{}).b == 0.0);
 }
+
+TEST_CASE("PostIndexingRefinement_CoplanarCandidateIsRejected", "[Indexing]") {
+    // A candidate whose three rows lie in one plane passes every length and angle test - three rows
+    // 120 deg apart in a plane meet the 30-150 deg window - and every spot along the plane normal
+    // indexes as (0,0,0), so the spot count passes too. Such a cell has no reciprocal basis; handed
+    // on, it made the online analysis throw "Crystal lattice is coplanar" for the frame instead of
+    // leaving the frame unindexed.
+    const Coord a(50, 0, 0);
+    const Coord b(-25, 25.0f * std::sqrt(3.0f), 0);
+    const Coord c(-25, -25.0f * std::sqrt(3.0f), 0);
+    REQUIRE(CrystalLattice(a, b, c).VolumeFraction() < MIN_BASIS_VOLUME_FRACTION);
+
+    std::vector<Coord> spots;
+    for (int i = 1; i <= 40; i++)
+        spots.emplace_back(0.0f, 0.0f, 0.01f * static_cast<float>(i));
+
+    Eigen::MatrixX3<float> cells(3, 3);
+    cells << a.x, a.y, a.z,
+             b.x, b.y, b.z,
+             c.x, c.y, c.z;
+    // A score below the refinement's distance floor, so the candidate reaches the filters as given.
+    Eigen::VectorX<float> scores(1);
+    scores << -10.99f;
+
+    RefineParameters parameters{
+        .viable_cell_min_spots = 9,
+        .dist_tolerance_vs_reference = 0.05,
+        .reference_unit_cell = std::nullopt,
+        .min_length_A = 5,
+        .max_length_A = 500,
+        .min_angle_deg = 30,
+        .max_angle_deg = 150,
+        .indexing_tolerance = 0.1
+    };
+
+    const auto lattices = Refine(spots, spots.size(), cells, scores, parameters);
+    CHECK(lattices.empty());
+    for (const auto &l: lattices)
+        CHECK_NOTHROW(l.GetUBMatrix());
+}