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.

dataflow.yml

@@ -19,6 +19,7 @@ nodes:
       tick: dora/timer/millis/33
     outputs:
       - image
+      - depth
       - bbox
       - text
 
@@ -27,5 +28,6 @@ nodes:
     build: pip install .
     inputs:
       image: source/image
+      depth: source/depth
       bbox: source/bbox
       text: source/text