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/plot.py

@@ -0,0 +1,72 @@
+"""The display state: the frame, the overlays drawn on it, and the window."""
+
+from dataclasses import dataclass, field
+
+import cv2
+import numpy as np
+
+WINDOW = "Dora Node: opencv-plot"
+GREEN = (0, 255, 0)
+WHITE = (255, 255, 255)
+FONT = cv2.FONT_HERSHEY_SIMPLEX
+FONT_SCALE = 0.5
+
+
+@dataclass
+class Bboxes:
+    """One overlay's worth of boxes, always stored as xyxy corners."""
+
+    corners: np.ndarray = field(default_factory=lambda: np.empty((0, 4)))
+    conf: np.ndarray = field(default_factory=lambda: np.empty(0))
+    labels: np.ndarray = field(default_factory=lambda: np.empty(0, dtype=object))
+
+
+@dataclass
+class Plot:
+    """Overlay state, plus the window it is displayed in.
+
+    `headless` suppresses the window; the node still decodes every input.
+    """
+
+    width: int | None = None
+    height: int | None = None
+    headless: bool = False
+    bboxes: Bboxes = field(default_factory=Bboxes)
+    text: str = ""
+
+    def show(self, frame: np.ndarray) -> bool:
+        """Draw the overlays on `frame`, resize it, and display it.
+
+        Returns whether the viewer asked to quit by pressing `q`.
+        """
+        self._draw(frame)
+
+        if self.width and self.height:
+            frame = cv2.resize(frame, (self.width, self.height))
+
+        if self.headless:
+            return False
+
+        cv2.imshow(WINDOW, frame)
+        return cv2.waitKey(1) & 0xFF == ord("q")
+
+    def _draw(self, frame: np.ndarray) -> None:
+        for corners, conf, label in zip(
+            self.bboxes.corners,
+            self.bboxes.conf,
+            self.bboxes.labels,
+            strict=False,
+        ):
+            min_x, min_y, max_x, max_y = (int(value) for value in corners)
+            cv2.rectangle(frame, (min_x, min_y), (max_x, max_y), GREEN, 2)
+            cv2.putText(
+                frame,
+                f"{label}, {conf:0.2f}",
+                (max_x - 120, max_y - 10),
+                FONT,
+                FONT_SCALE,
+                GREEN,
+                1,
+            )
+
+        cv2.putText(frame, self.text, (20, 20), FONT, FONT_SCALE, WHITE, 1)