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/dora-node.yml

@@ -12,7 +12,7 @@
 apiVersion: 1
 name: source
 namespace: liria
-description: Synthetic image, bbox and text source for the dora-image-view example
+description: Synthetic image, depth, bbox and text source for the dora-image-view example
 categories: [debug]
 keywords: [example, test, synthetic]
 
@@ -39,6 +39,12 @@ outputs:
   text:
     type: std/core/v1/String
     description: A fixed caption.
+  depth:
+    type: std/core/v1/Float32
+    description: >-
+      Depth of the same scene in millimetres, [height, width]: a flat wall with
+      the square standing in front of it.
+
 
 env:
   SOURCE_WIDTH: