grid scan: an unstated beam is reported as the grid step, not as zero

A raster is stepped at about the beam size, so the step is the best proxy there is
when neither --beam-size nor the file's incident_beam_size says anything. Zero is the
worse answer: the reported crystal extents still contain a whole beam, and a zero
tells a consumer deconvolving them that they are already exact.

The substitution is in AnalyzeGridScan, so it holds for the broker and for rugnux
alike. The extents themselves do not move - they are measured either way, and the
test pins that. BEAM_SIZE_SOURCE in the raster report gains GRID_STEP, so a reader
can still tell a measured beam from a stood-in one, which matters because removing an
anisotropic beam is a covariance subtraction and a wrong one rotates the crystal axis.

The comment at the rugnux call site had argued for the old behaviour in as many
words; it now describes what the code does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EFEJG6WBQv8th4UJFNe53N

tests/AnalyzeGridScanTest.cpp

@@ -331,3 +331,26 @@ TEST_CASE("AnalyzeGridScan follows the snake and the step signs", "[AnalyzeGridS
     CHECK(a.y_um == Catch::Approx(b.y_um));
     CHECK(snake.Rearrange(b.image_number) == plain.Rearrange(a.image_number));
 }
+
+TEST_CASE("AnalyzeGridScan falls back to the grid step for an unstated beam", "[AnalyzeGridScan]") {
+    auto grid = MakeGrid(8, 8, 20.0f, 16.0f);
+    const std::vector<std::pair<int64_t, int64_t>> blob{{2, 2}, {3, 2}, {2, 3}, {3, 3}};
+    const auto scan = MakeScan(8, 8, blob, 0.9f);
+
+    // Stated: echoed back untouched, whatever the step is.
+    const auto stated = AnalyzeGridScan(scan, grid, 80.0f, 20.0f);
+    CHECK(stated.beam_size_x_um == Catch::Approx(80.0));
+    CHECK(stated.beam_size_y_um == Catch::Approx(20.0));
+
+    // Unstated: the step stands in. Zero would tell a consumer the extents are exact when they
+    // still contain a whole beam, and the two axes must not collapse to one value.
+    const auto unstated = AnalyzeGridScan(scan, grid, 0.0f, 0.0f);
+    CHECK(unstated.beam_size_x_um == Catch::Approx(20.0));
+    CHECK(unstated.beam_size_y_um == Catch::Approx(16.0));
+
+    // The extents themselves are measured and must not move with the beam.
+    REQUIRE(stated.crystals.size() == 1);
+    REQUIRE(unstated.crystals.size() == 1);
+    CHECK(stated.crystals[0].major_um == Catch::Approx(unstated.crystals[0].major_um));
+    CHECK(stated.crystals[0].minor_um == Catch::Approx(unstated.crystals[0].minor_um));
+}