add help docs and ?

docs/nodes/FFT 2D.md

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+# FFT 2D
+
+Compute the 2D FFT with optional windowing and mean/plane subtraction. Outputs log magnitude, magnitude, phase, and PSDF as separate channels. Equivalent to gwy_data_field_2dfft / gwy_data_field_2dpsdf.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input spatial-domain field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| log_magnitude | DATA_FIELD | log(1 + |F|) of the 2D FFT, centered on DC |
+| magnitude | DATA_FIELD | |F| of the 2D FFT, centered on DC |
+| phase | DATA_FIELD | Phase angle of the 2D FFT in radians, centered on DC |
+| psdf | DATA_FIELD | 2D power spectral density function |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| windowing | dropdown | hann | Window function applied before FFT: hann, hamming, blackman, or none |
+| level | dropdown | mean | Pre-processing: subtract mean, subtract plane, or none |
+
+## Limitations
+
+- Output fields are in the frequency domain; physical units on axes are spatial frequency (1/m).
+- Phase output uses the raw FFT phase and is sensitive to field origin; it is most useful when paired with the Inverse 2D FFT node.