Fix final-review findings: pixel-scale size guard, DRY camera field names

effective_pixel_scale now raises a clear ValueError for image shapes
too small for its finite-difference indexing, instead of an IndexError.
ModalFitter.fit()'s geometry dict now reuses CAMERA_FIELD_NAMES from
geometry.py instead of a duplicated hardcoded tuple.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

he11lib/fitting.py

@@ -9,6 +9,7 @@ from scipy.optimize import least_squares
 
 from .data import MeasurementPlane, ReconstructionResult, validate_planes
 from .geometry import (
+    CAMERA_FIELD_NAMES,
     CameraModel,
     CameraModelTolerance,
     GeometryCalibration,
@@ -166,14 +167,7 @@ class ModalFitter:
             for i in range(len(planes))
         ]
 
-        geometry: dict[str, float] = dict(zip(
+        geometry: dict[str, float] = dict(zip(CAMERA_FIELD_NAMES, camera_to_values(fitted_camera)))
-            (
-                "focal_length_px", "position_x", "position_y", "position_z",
-                "yaw_deg", "pitch_deg", "roll_deg",
-                "principal_point_x", "principal_point_y",
-            ),
-            camera_to_values(fitted_camera),
-        ))
         for i in range(len(planes)):
             geometry[f"z_{i}"] = z_values[i]
 

he11lib/geometry.py

@@ -237,6 +237,11 @@ class GeometryCalibration:
         keystoned.
         """
         rows, cols = image_shape
+        if rows < 2 or cols < 2:
+            raise ValueError(
+                f"image_shape must be at least 2x2 to compute a finite-difference "
+                f"pixel scale, got {image_shape}"
+            )
         x, y = self.physical_coordinates(image_shape, z)
         mid_row, mid_col = rows // 2, cols // 2
         dx = abs(x[mid_row, mid_col + 1] - x[mid_row, mid_col])

tests/test_geometry.py

@@ -149,3 +149,12 @@ def test_effective_pixel_scale_matches_on_axis_focal_length():
     scale = calib.effective_pixel_scale((41, 41), z)
     expected = (z - camera_z) / focal_length_px
     assert scale == pytest.approx(expected, rel=1e-6)
+
+
+def test_effective_pixel_scale_raises_for_image_too_small():
+    camera = make_on_axis_camera()
+    calib = GeometryCalibration(camera)
+    z = 0.5
+
+    with pytest.raises(ValueError, match="image_shape must be at least 2x2"):
+        calib.effective_pixel_scale((1, 1), z)