Initial commit: he11lib mode-purity reconstruction library

Full implementation of Laguerre-Gauss modal reconstruction for gyrotron
beam diagnostics, per the approved design spec, plus tests, docs, and
a runnable end-to-end example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_plotting.py

@@ -0,0 +1,64 @@
+import matplotlib.figure
+import numpy as np
+import pytest
+
+from he11lib.data import MeasurementPlane, ReconstructionResult
+from he11lib.plotting import plot_center_trace, plot_mode_purity, plot_residuals
+
+
+def make_result(**overrides):
+    defaults = dict(
+        purity={(0, 0): (0.9, 0.1), (1, 0): (0.1, -0.2)},
+        reconstructed_field=np.zeros((5, 5), dtype=complex),
+        centers=[(0.0, 0.0), (1e-4, -1e-4), (2e-4, -2e-4)],
+        pointing_angle_deg=0.5,
+        residuals=[np.ones((5, 5)), np.ones((5, 5)) * 2, np.ones((5, 5)) * 3],
+    )
+    defaults.update(overrides)
+    return ReconstructionResult(**defaults)
+
+
+def make_planes():
+    return [
+        MeasurementPlane(flux=np.zeros((5, 5)), z=0.3),
+        MeasurementPlane(flux=np.zeros((5, 5)), z=0.5),
+        MeasurementPlane(flux=np.zeros((5, 5)), z=0.7),
+    ]
+
+
+def test_plot_mode_purity_draws_one_bar_per_mode():
+    result = make_result()
+    fig = plot_mode_purity(result)
+
+    assert isinstance(fig, matplotlib.figure.Figure)
+    ax = fig.axes[0]
+    assert len(ax.patches) == len(result.purity)
+
+
+def test_plot_center_trace_plots_one_point_per_plane():
+    planes = make_planes()
+    result = make_result()
+    fig = plot_center_trace(planes, result)
+
+    assert isinstance(fig, matplotlib.figure.Figure)
+    ax = fig.axes[0]
+    line = ax.lines[0]
+    assert len(line.get_xdata()) == len(planes)
+
+
+def test_plot_residuals_draws_one_axes_per_plane():
+    planes = make_planes()
+    result = make_result()
+    fig = plot_residuals(planes, result)
+
+    assert isinstance(fig, matplotlib.figure.Figure)
+    image_axes = [ax for ax in fig.axes if ax.images]
+    assert len(image_axes) == len(planes)
+
+
+def test_plot_residuals_raises_when_phase_retrieval_used_without_residuals():
+    planes = make_planes()
+    result = make_result(residuals=[], used_phase_retrieval=True)
+
+    with pytest.raises(ValueError, match="residuals"):
+        plot_residuals(planes, result)