add help docs and ?

docs/nodes/Grain Filter.md

@@ -0,0 +1,28 @@
+# Grain Filter
+
+Remove grains from a binary mask based on size and border contact. Equivalent to Gwyddion's grain_filter module (grain_filter.c).
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| mask | IMAGE | Yes | Binary mask containing grain regions to filter |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| filtered_mask | IMAGE | Binary mask with unwanted grains removed |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| min_area | INT | 10 | Discard grains with fewer pixels than this value (0 = no lower limit) |
+| max_area | INT | 0 | Discard grains with more pixels than this value (0 = no upper limit) |
+| remove_border | BOOLEAN | False | Discard any grain that touches the image edge |
+
+## Limitations
+
+- Grain detection uses 4-connected or 8-connected labeling; the exact connectivity is determined by the implementation.
+- min_area and max_area are in pixels, not physical units.