adding more nodes

docs/nodes/Flatten Base.md

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+# Flatten Base
+
+Level the flat base of a surface that has raised features (particles, grains). Uses a height percentile threshold to identify base pixels, fits a polynomial to those pixels, and subtracts it. Unlike plane level, this ignores tall features that would bias the fit. Equivalent to Gwyddion's flatten_base.c module.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field with raised features on a tilted base |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| leveled | DATA_FIELD | Field with base leveled |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| threshold_percentile | FLOAT | 30.0 | Height percentile below which pixels are considered base (5–80) |
+| poly_degree | INT | 2 | Polynomial degree for base fit: 0 = constant, 1 = plane, 2 = quadratic (0–5) |
+
+## Notes
+
+- Set the threshold percentile so that it includes most of the base but excludes the features. For sparse particles on a flat substrate, 20–40% typically works well.
+- poly_degree=1 is equivalent to plane leveling on the base only. Use 2–3 for curved substrates.
+- If the features dominate the surface (>50% coverage), this node may not give good results — consider Median Background instead.