add help docs and ?

docs/nodes/Radial Profile.md

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+# Radial Profile
+
+Compute the azimuthally averaged radial profile from a centre point. The output x-axis is radius in physical xy units. Equivalent to gwy_data_field_angular_average used by Gwyddion's Radial Profile tool.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| profile | LINE | Radially averaged profile vs. radius |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| cx | FLOAT | 0.5 | Centre x position as a fraction of field width (0 = left, 1 = right) |
+| cy | FLOAT | 0.5 | Centre y position as a fraction of field height (0 = top, 1 = bottom) |
+| n_bins | INT | 128 | Number of radial bins (4–4096) |
+
+## Limitations
+
+- Pixels are assigned to radial bins by Euclidean distance; bins near the centre contain fewer pixels and may be noisier.
+- Physical x-axis units come from the field's si_unit_xy; uncalibrated fields produce pixel-unit radii.