add a depth map panel

The node showed a colour image only, so debugging a depth pipeline meant
either trusting the numbers or writing a throwaway viewer. It now keeps one
panel per frame stream -- image and depth -- and tiles them side by side,
scaled to a common height. Wiring one is a plain viewer; wiring both shows
them together. bbox and text stay overlays rather than panels.

The contract follows what dora-zed already publishes: float32 [H, W] in
millimetres with 0 for missing. It is sent there as a raw byte slice, so the
payload arrives as UInt8 despite the Float32 contract and is reinterpreted
rather than converted.

Auto-ranging the colormap over the valid pixels' full extent, rather than over
a percentile of them, is deliberate and was not the first attempt: trimming
the tails collapsed on the scene the node exists for. A small object in front
of a large wall is a few percent of the pixels, so both tails landed on the
wall and the object clipped away. Rendering a frame and looking at it found
that; every test passed either way.

Verified with dora-cli 1.0.1: validate passes with both contracts injected,
the dataflow runs both nodes to a clean exit, and a composed frame was
rendered and inspected.

examples/source.py

@@ -1,8 +1,11 @@
 """Synthetic source node: this repository's stand-in for a camera and a detector.
 
-Emits an animated frame, the box around the square moving in it, and a caption --
-the three inputs dora-image-view draws -- so the example dataflow needs no camera, no
-detector and no network.
+Emits an animated frame, a depth map of the same scene, the box around the square
+moving in it, and a caption -- every input dora-image-view draws -- so the example
+dataflow needs no camera, no detector and no network.
+
+The depth map describes the same scene as the image: a flat back wall with the square
+standing in front of it.
 """
 
 import os
@@ -21,9 +24,12 @@ BOX = 96
 SPEED = 6
 CAPTION = "dora-image-view synthetic source"
 
+WALL_MM = 2000.0
+SQUARE_MM = 800.0
+
 
 def main() -> None:
-    """Send one image, bbox and text per tick."""
+    """Send one image, depth map, bbox and text per tick."""
     node = Node("source")
     sent = 0
 
@@ -46,6 +52,12 @@ def main() -> None:
             }]),
             {"format": "xyxy"},
         )
+        depth = render_depth(corners)
+        node.send_output(
+            "depth",
+            pa.array(depth.ravel()),
+            {"shape": [HEIGHT, WIDTH], "dtype": "float32", "layout": "HW", "units": "mm"},
+        )
         node.send_output("text", pa.array([CAPTION]))
 
         sent += 1
@@ -69,5 +81,13 @@ def render(index: int) -> tuple[np.ndarray, np.ndarray]:
     return frame, corners
 
 
+def render_depth(corners: np.ndarray) -> np.ndarray:
+    """Depth of the scene in `render`: a flat wall with the square in front of it."""
+    depth = np.full((HEIGHT, WIDTH), WALL_MM, dtype=np.float32)
+    left, top, right, bottom = (int(value) for value in corners)
+    depth[top:bottom, left:right] = SQUARE_MM
+    return depth
+
+
 if __name__ == "__main__":
     main()