add help docs and ?

docs/nodes/Fractal Dimension.md

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+# Fractal Dimension
+
+Calculate the surface fractal dimension using Gwyddion's partitioning, cube counting, triangulation, power-spectrum, or HHCF methods. The in-node graph shows the log-log curve and allows dragging the fit range.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input surface field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| dimension | FLOAT | Estimated fractal dimension |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| method | dropdown | partitioning | Algorithm: partitioning, cube_counting, triangulation, psdf (power spectrum), or hhcf (structure function) |
+| interpolation | dropdown | linear | Interpolation used when resampling the field to a square grid: linear, nearest, or cubic |
+
+## Limitations
+
+- The field is resampled to a square grid internally; highly anisotropic scan sizes may introduce interpolation artefacts.
+- Fit range can be adjusted interactively on the log-log plot; the default range covers the full data.