add help docs and ?

docs/nodes/Coordinate.md

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+# Coordinate
+
+Output a fractional (x, y) coordinate pair in [0, 1] for use with Cross Section, Cursors, or other nodes that accept a COORD input.
+
+## Inputs
+
+None.
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| point | COORD | Fractional (x, y) coordinate |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| x | FLOAT | 0.5 | Horizontal position as a fraction of image width (0 = left, 1 = right) |
+| y | FLOAT | 0.5 | Vertical position as a fraction of image height (0 = top, 1 = bottom) |
+
+## Limitations
+
+- Values are clamped to [0, 1] by downstream nodes; this node does not enforce clamping itself.