add help docs and ?

docs/nodes/Stats.md

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+# Stats
+
+Compute a contextual scalar statistic from a LINE, record table, DATA_FIELD, or IMAGE. The available operations adapt to the connected input type.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| input | DATA_FIELD, IMAGE, LINE, or DATA_TABLE | Yes | Input to compute statistics on |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| value | FLOAT | Computed scalar statistic |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| operation | STRING (context-dependent dropdown) | mean | Statistical operation; available choices depend on the input type |
+| column | STRING | value | Column name for DATA_TABLE inputs; visible only when a DATA_TABLE is connected |
+
+## Limitations
+
+- Available operations vary by input type; connecting a different type changes the available options.
+- For DATA_TABLE inputs, the column must match an existing column name; an error is raised otherwise.