add help docs and ?

docs/nodes/Facet Level.md

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+# Facet Level
+
+Level a field by iteratively finding the dominant local facet orientation and subtracting the corresponding plane. Matches Gwyddion's facet-level behaviour. Supports mask include/exclude selection.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input topographic field |
+| mask | IMAGE | No | Binary mask for including or excluding facets from plane fitting |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| leveled | DATA_FIELD | Field with dominant facet plane subtracted |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| masking | dropdown | exclude | How to use the mask: exclude (ignore masked facets), include (use only masked facets), or ignore (use all facets) |
+
+## Limitations
+
+- Requires compatible XY and Z physical units for correct facet gradient estimation.
+- Needs at least four valid facet cells; fields smaller than 2×2 pixels are not processed.
+- Flat surfaces with no variation in facet orientation may not converge to a meaningful plane.