add help docs and ?

docs/nodes/FFT Filter.md

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+# FFT Filter
+
+Frequency-domain filtering of a line profile or 2D data field using a Butterworth transfer function. Connect a LINE for 1D filtering or a DATA_FIELD for 2D filtering; the output mirrors the input type. Equivalent to Gwyddion fft_filter_1d / fft_filter_2d.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| input | LINE or DATA_FIELD | Yes | Input line or field to filter |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| filtered | LINE or DATA_FIELD | Filtered output matching the input type |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| filter_type | dropdown | lowpass | Filter mode: lowpass, highpass, bandpass, or notch (band-reject) |
+| cutoff | FLOAT | 0.1 | Lower cutoff frequency as a fraction of Nyquist (0.001–1.0) |
+| cutoff_high | FLOAT | 0.4 | Upper cutoff for bandpass/notch modes (0.001–1.0) |
+| order | INT | 2 | Butterworth filter order; higher values give steeper roll-off (1–10) |
+
+## Limitations
+
+- cutoff_high is only used for bandpass and notch modes.
+- The filter is applied in the frequency domain via FFT; very short lines may show wrap-around artefacts.