use std/media/v1/Image for image and depth

Both inputs now declare dora's Arrow struct {width, height, encoding, data}
rather than a flat buffer plus loose metadata. The payload describes itself, so
nothing about its shape has to be agreed out of band, and the frame travels as
one Arrow value instead of a buffer whose meaning lives in a sidecar dict.

Depth uses the same type. dora's registry has no depth type, and a depth map is
an image whose pixels are distances -- the reading ROS takes as well -- so it
arrives with a single-channel encoding: 32FC1/float32 or 16UC1/mono16/uint16.
`units` stays in message metadata because the struct has no field for it.

decode.IMAGE_TYPE is the single definition of the struct; the example source
and the tests build payloads from it rather than respelling the schema.

The older shapes are still decoded -- a flat UInt8 buffer with
width/height/encoding for image, a flat Float32 buffer with shape/units for
depth -- so a producer's payload can move before its manifest does. That is
only half a migration path: type compatibility is directional, so a producer
still declaring std/core/v1/UInt8 fails validate against an
std/media/v1/Image input regardless of what it sends, and has to update its
manifest too.

Verified: 41 tests, validate clean with 4 contracts injected, the dataflow runs
to a clean exit, and a composite rendered through the struct path is identical
to the one rendered before the migration.

dora-node.yml

@@ -21,13 +21,14 @@ dora: ">=1.0"
 
 inputs:
   image:
-    type: std/core/v1/UInt8
+    type: std/media/v1/Image
     required: false
     description: >-
-      Base image to draw on: a flat UInt8 buffer described by the metadata
-      {width, height, encoding}. Supported encodings: bgr8, rgb8,
-      jpeg/jpg/jpe/bmp/webp/png (encoded bytes), yuv420, avif. Receiving this
-      input triggers a redraw and display.
+      Base image to draw on: an Arrow struct {width, height, encoding, data}.
+      Supported encodings: bgr8, rgb8, jpeg/jpg/jpe/bmp/webp/png (encoded
+      bytes), yuv420, avif. Receiving this input triggers a redraw and display.
+      A flat UInt8 buffer described by {width, height, encoding} metadata is
+      still decoded, for producers that have not moved to the struct.
   bbox:
     required: false
     description: >-
@@ -53,13 +54,17 @@ inputs:
       this input -- they are picked up from there, which is what covers a
       camera that published its intrinsics before this node started.
   depth:
-    type: std/core/v1/Float32
+    type: std/media/v1/Image
     required: false
     description: >-
-      Depth map as a flat Float32 buffer, described by the metadata
-      {shape: [height, width], units}. Millimetres unless `units` says `m`;
-      0 and non-finite values are missing depth and draw black. Colour-mapped
-      into its own panel, which receiving this input redraws.
+      Depth map as an Arrow struct {width, height, encoding, data}, one channel
+      per pixel. Supported encodings: 32FC1/float32 and 16UC1/mono16/uint16 --
+      dora has no depth type, and a depth map is an image whose pixels are
+      distances, which is how ROS spells it too. Millimetres unless the `units`
+      metadata says `m`; 0 and non-finite values are missing depth and draw
+      black. Colour-mapped into its own panel, which receiving this input
+      redraws. A flat Float32 buffer with {shape, units} metadata is still
+      decoded, for producers that have not moved to the struct.
 
 outputs: {}