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.

dora-node.yml

@@ -0,0 +1,64 @@
+# Node manifest: the typed contract a consumer wires against.
+#
+# No source package is shared between node repositories, so this file and its
+# type URNs are the only contract a consumer has. `dora validate` checks a
+# dataflow against it without running the node, so CI needs neither OpenCV nor
+# a display.
+apiVersion: 1
+name: opencv-plot
+namespace: liria
+description: Overlays bounding boxes, labels, and text on an image input and displays it in an OpenCV window
+categories: [visualization, debug]
+keywords: [opencv, plot, bbox, bounding-box, image, visualization, object-detection]
+
+runtime: python
+entrypoint: opencv-plot
+# `dora build --uv` / `dora run --uv` rewrites this to `uv pip install .` and
+# gives the node its own managed venv under `.dora/python-envs/`, so the
+# dependencies it sees at runtime are the ones installed here.
+build: pip install .
+dora: ">=1.0"
+
+inputs:
+  image:
+    type: std/core/v1/UInt8
+    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.
+  bbox:
+    required: false
+    description: >-
+      Bounding boxes to overlay: an Arrow struct with fields {bbox, conf,
+      labels} and metadata {format}. Supported formats: xyxy, xywh. Stored and
+      drawn on the next image redraw. Deliberately untyped — senders differ in
+      whether the numeric fields are 32- or 64-bit, and the node casts either.
+  text:
+    type: std/core/v1/String
+    required: false
+    description: >-
+      Text string to overlay at the top-left corner. Stored and drawn on the
+      next image redraw.
+
+outputs: {}
+
+env:
+  PLOT_WIDTH:
+    type: int
+    description: Resize the displayed frame to this width (pixels). Defaults to the image input width.
+  PLOT_HEIGHT:
+    type: int
+    description: Resize the displayed frame to this height (pixels). Defaults to the image input height.
+
+example: |
+  - id: plot
+    git: ssh://git@gitea.liria.cl:5001/liria/dora-plot.git
+    tag: v0.1.0
+    build: pip install .
+    path: opencv-plot
+    inputs:
+      image: camera/image
+      bbox: detector/bbox
+      text: detector/text