add a few more nodes

docs/nodes/Zero Mean.md

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+# Zero Mean
+
+Shift all values so the mean is exactly zero. A pure offset subtraction — no plane fit or polynomial involved.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to level |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| leveled | DATA_FIELD | Field with mean subtracted |
+
+## Notes
+
+- Equivalent to subtracting a constant (the mean) from every pixel.
+- Does not change relative height differences — only shifts the overall offset.