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>

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)