add help docs and ?

docs/nodes/ACF 2D.md

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+# ACF 2D
+
+Compute the two-dimensional autocorrelation function with Gwyddion-style mean or plane levelling before correlation. The output is centered on zero shift and uses default half-range extents.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input height field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| acf | DATA_FIELD | 2D autocorrelation field centered on zero shift |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| level | dropdown | mean | Pre-processing applied before correlation: mean subtraction, plane subtraction, or none |
+
+## Limitations
+
+- Output is not normalized to [−1, 1]; peak value equals the field variance.
+- Plane levelling assumes a linear trend; strongly curved surfaces may not detrend correctly.