add help docs and ?

docs/nodes/FFT 1D.md

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+# FFT 1D
+
+Compute the FFT amplitude spectrum of a line profile and identify the dominant period. The output x-axis is period (not frequency), sorted from short to long.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| profile | LINE | Yes | Input line profile |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| frequency_plot | LINE | FFT amplitude spectrum vs. period |
+| max | RECORD_TABLE | Table with the peak period |
+
+## Controls
+
+None.
+
+## Limitations
+
+- The DC component is excluded from the output.
+- Spectrum is one-sided (real FFT); the x-axis shows period, not frequency.
+- No windowing is applied; spectral leakage may affect results on non-periodic inputs.