add help docs and ?

docs/nodes/Custom Convolution.md

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+# Custom Convolution
+
+Apply a user-defined convolution kernel to a DATA_FIELD. Enter rows of space-separated numbers. Equivalent to Gwyddion convolution_filter.c.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to filter |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| result | DATA_FIELD | Filtered field |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| kernel | STRING (multiline) | "0 -1 0\n-1 5 -1\n0 -1 0" | Kernel rows, each as space-separated numbers on a new line |
+| normalize | BOOLEAN | True | Divide the result by the sum of absolute kernel values to preserve amplitude |
+| boundary | dropdown | reflect | Boundary handling: reflect, nearest, or wrap |
+
+## Limitations
+
+- Kernel must be a rectangle: all rows must have the same number of values.
+- Maximum kernel size is 51×51; larger kernels are rejected.
+- Non-finite values in the kernel are rejected and the input is returned unchanged.
+- For large kernels, direct convolution can be slow; consider FFT Filter instead.