Do not let a marCCD file name 2-theta as the scanned circle

The scanned circle was read as the first start/end pair that differs,
and two_theta is the first pair in the goniostat block. A file that
leaves its end block at zero while the detector sits on a swung-out
2-theta arm therefore named the arm as the spindle and stopped before
reaching phi; with no rotation range recorded either, the increment fell
back to -start_angle, which differs per frame, and the sweep was refused
outright.

The header's own rotation_axis index (offset 732) is taken where it
names a spindle circle, and the pair scan is the fallback for files that
never filled it in - read past two_theta, which no sweep turns.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nW6FNRP1bBJJ8pfHiByAT

reader/MarCCD.cpp

@@ -38,6 +38,7 @@ constexpr size_t OFF_END_XTAL_TO_DETECTOR = 728;     // ... and at its end
 constexpr size_t OFF_EXPOSURE_TIME = 656;      // milliseconds
 constexpr size_t OFF_START_TWOTHETA = 668;     // millidegrees, and the seven that follow it
 constexpr size_t OFF_END_TWOTHETA = 700;       // the same seven, at the end of the exposure
+constexpr size_t OFF_ROTATION_AXIS = 732;      // which circle the sweep turned, as an index below
 constexpr size_t OFF_ROTATION_RANGE = 736;     // millidegrees
 constexpr size_t OFF_PIXELSIZE_X = 772;        // nanometres
 constexpr size_t OFF_PIXELSIZE_Y = 776;
@@ -146,21 +147,33 @@ Header Parse(const std::vector<uint8_t> &h, const std::string &path) {
     out.exposure_s = I32(h, OFF_EXPOSURE_TIME) * 1e-3;
     out.saturated_value = I32(h, OFF_SATURATED);
 
-    // Which circle moved. The header also carries a rotation_axis index, but the start/end pair
-    // that differs is the same answer read off the angles themselves, and it stays right on a file
-    // whose index field was never filled in.
+    // Which circle moved. The header names it as an index into the circle list, and that is taken
+    // where it names a spindle circle: reading the start/end pairs alone takes the FIRST pair that
+    // differs, and two_theta is the first of them, so a file that left its end block at zero while
+    // the detector sat on a swung-out 2-theta arm had the arm named as the spindle - and, with no
+    // rotation range recorded either, an increment of -start_angle that differs on every frame,
+    // which SweepLayout then refuses the whole sweep over.
+    //
+    // The index is not always filled in, and a file that leaves it at zero cannot be told from one
+    // that means two_theta by it, so the fallback is the pairs as before - read past two_theta,
+    // which is a detector circle and never the one a sweep turns.
     out.angle_increment_deg = I32(h, OFF_ROTATION_RANGE) * 1e-3;
-    for (size_t i = 0; i < N_CIRCLES; i++) {
+    const auto take_circle = [&](size_t i) {
         const double start = I32(h, OFF_START_TWOTHETA + 4 * i) * 1e-3;
         const double end = I32(h, OFF_END_TWOTHETA + 4 * i) * 1e-3;
-        if (start != end) {
-            out.axis_name = CIRCLE_NAMES[i];
-            out.start_angle_deg = start;
-            if (out.angle_increment_deg == 0.0)
-                out.angle_increment_deg = end - start;
-            break;
-        }
-    }
+        if (start == end)
+            return false;
+        out.axis_name = CIRCLE_NAMES[i];
+        out.start_angle_deg = start;
+        if (out.angle_increment_deg == 0.0)
+            out.angle_increment_deg = end - start;
+        return true;
+    };
+    const int32_t named = I32(h, OFF_ROTATION_AXIS);
+    if (named <= 0 || static_cast<size_t>(named) >= N_CIRCLES || !take_circle(static_cast<size_t>(named)))
+        for (size_t i = 1; i < N_CIRCLES; i++)
+            if (take_circle(i))
+                break;
     return out;
 }