accept std/media/v1/Image payloads

dora's typed image is an Arrow struct of width, height, encoding and data --
it carries inside the payload what this node has been reading out of message
metadata. The decoder now takes either shape: a struct describes itself and
its metadata is ignored, a flat UInt8 buffer is described by width/height/
encoding beside it, and everything after unpacking is common.

The declared contract stays std/core/v1/UInt8, deliberately. Type
compatibility is directional -- Image satisfies a Bytes input, but a
std/core/v1/UInt8 output wired into an std/media/v1/Image input is a validate
error -- and dora-zed declares image_bgr as std/core/v1/UInt8. Flipping this
manifest now would break the camera. Accepting both shapes first is what lets
the producers move before the contract does.

output_framing is not part of this. It selects the wire format, raw or
arrow-ipc, and is not required for images: a struct payload was verified to
arrive identically under both framings, and validate does not ask for
arrow-ipc when an output is annotated std/media/v1/Image. It earns its keep
when the wire must be self-describing, such as crossing languages or
recording.

dora_image_view/main.py

@@ -67,12 +67,7 @@ def _decode_panel(event_id: str, value, metadata: dict) -> tuple[str, np.ndarray
     event loop to treat it as overlay state instead.
     """
     if event_id == "image":
-        return "image", decode.to_bgr(
-            value,
-            metadata.get("encoding", "bgr8").lower(),
-            metadata.get("width"),
-            metadata.get("height"),
-        )
+        return "image", decode.to_bgr(value, metadata)
 
     if event_id == "depth":
         return "depth", depth.colorize(depth.to_millimetres(value, metadata))