add opencv-plot node for dora dataflows

A display sink that overlays bounding boxes and text on an image input and
shows it in an OpenCV window. Based on opencv-plot from dora-hub, brought up
to the dora 1.0 node contract and packaged with uv.

Python rather than Rust: the opencv Rust crate generates its bindings from the
OpenCV headers on the build machine, so it needs libopencv-dev and libclang-dev
per machine and per architecture and does not cross-compile. Every dependency
here ships prebuilt wheels for linux-x86_64, linux-aarch64 and macos-arm64.

Covers every encoding the upstream node did — bgr8, rgb8, jpeg/jpg/jpe/bmp/
webp/png, yuv420 and avif — plus both bbox formats, the text overlay,
PLOT_WIDTH/PLOT_HEIGHT and headless operation under CI=true.

opencv_plot/main.py

@@ -0,0 +1,84 @@
+"""A dora display sink: overlays bounding boxes and text on an image input.
+
+An `image` event decodes the frame and immediately redraws. `bbox` and `text`
+events only update stored state, which the next `image` event draws.
+"""
+
+import argparse
+import os
+
+from dora import Node
+
+from opencv_plot import bbox, decode
+from opencv_plot.plot import Plot
+
+
+def main() -> None:
+    """Connect to the dataflow and display every frame that arrives."""
+    args = _parse_args()
+
+    plot = Plot(
+        width=_size("PLOT_WIDTH", args.plot_width),
+        height=_size("PLOT_HEIGHT", args.plot_height),
+        headless=os.getenv("CI") == "true",
+    )
+
+    # Passing the name lets the node also run standalone as a dynamic node; when
+    # dora spawns it, the id from the dataflow wins.
+    node = Node(args.name)
+
+    for event in node:
+        if event["type"] == "ERROR":
+            raise RuntimeError(event["error"])
+        if event["type"] != "INPUT":
+            continue
+
+        event_id = event["id"]
+        metadata = event["metadata"]
+
+        if event_id == "image":
+            frame = decode.to_bgr(
+                event["value"],
+                metadata.get("encoding", "bgr8").lower(),
+                metadata.get("width"),
+                metadata.get("height"),
+            )
+            if plot.show(frame):
+                break
+        elif event_id == "bbox":
+            plot.bboxes = bbox.parse(event["value"], metadata.get("format", "xyxy").lower())
+        elif event_id == "text":
+            plot.text = event["value"][0].as_py()
+        else:
+            print(f"ignoring unexpected input `{event_id}`")
+
+
+def _parse_args() -> argparse.Namespace:
+    parser = argparse.ArgumentParser(
+        description="OpenCV plotter: draws text and bounding boxes on an image.",
+    )
+    parser.add_argument(
+        "--name",
+        default="opencv-plot",
+        help="The name of the node in the dataflow, used when running as a dynamic node.",
+    )
+    parser.add_argument(
+        "--plot-width",
+        type=int,
+        help="Resize the displayed frame to this width. Overridden by PLOT_WIDTH.",
+    )
+    parser.add_argument(
+        "--plot-height",
+        type=int,
+        help="Resize the displayed frame to this height. Overridden by PLOT_HEIGHT.",
+    )
+    return parser.parse_args()
+
+
+def _size(name: str, fallback: int | None) -> int | None:
+    value = os.getenv(name)
+    return int(value) if value and value.isnumeric() else fallback
+
+
+if __name__ == "__main__":
+    main()