add help docs and ?

docs/nodes/PSDF.md

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+# PSDF
+
+Compute the two-dimensional power spectral density function with Gwyddion-style window RMS compensation and centered zero frequency. Equivalent to psdf2d / gwy_data_field_2dpsdf.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input spatial-domain field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| psdf | DATA_FIELD | 2D power spectral density, centered on DC |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| windowing | dropdown | hann | Window function applied before FFT to reduce spectral leakage: hann, hamming, blackman, or none |
+| level | dropdown | mean | Pre-processing: subtract mean, subtract plane, or none |
+
+## Limitations
+
+- Output is in the frequency domain; physical units on axes are spatial frequency (1/m) and PSDF units are z_unit²·m².
+- Window RMS compensation is applied to normalize the spectral density consistently across window choices.