add help docs and ?

docs/nodes/Threshold Mask.md

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+# Threshold Mask
+
+Create a binary mask by thresholding data. Otsu automatically finds the optimal threshold. Equivalent to Gwyddion's threshold and otsu_threshold modules.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to threshold |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| mask | IMAGE | Binary mask (white = selected pixels) |
+| threshold | RECORD_TABLE | Table with the threshold value applied |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| method | dropdown | absolute | Thresholding method: absolute (raw data value), relative (fraction of min–max range), or otsu (automatic Otsu threshold) |
+| threshold | FLOAT | 0.0 | Threshold value; for absolute: raw z value; for relative: fraction 0–1; ignored for otsu (socket-only input) |
+| direction | dropdown | above | Which pixels to select: above or below the threshold |
+
+## Limitations
+
+- For the relative method, the threshold fraction is applied to the full data range [min, max].
+- Otsu thresholding may not give meaningful results on non-bimodal height distributions.