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.

pyproject.toml

@@ -0,0 +1,43 @@
+[project]
+name = "opencv-plot"
+version = "0.1.0"
+description = "Dora node that overlays bounding boxes and text on an image and displays it in an OpenCV window"
+readme = "README.md"
+license = { text = "MIT" }
+requires-python = ">=3.11"
+
+# Every dependency ships prebuilt wheels for linux-x86_64, linux-aarch64 and
+# macos-arm64, so the node installs with no system packages and no compiler on
+# any of them. That is the whole reason this node is Python and not Rust: the
+# OpenCV Rust bindings generate themselves from installed headers and would put
+# libopencv-dev and libclang-dev on every build machine and every arch.
+dependencies = [
+  "dora-rs >= 1.0.1",
+  "numpy >= 2.0",
+  "opencv-python >= 4.9",
+  "pillow >= 10.4",
+  "pillow-avif-plugin >= 1.4",
+]
+
+[project.scripts]
+opencv-plot = "opencv_plot.main:main"
+
+[dependency-groups]
+dev = ["pytest >= 8.1", "ruff >= 0.9"]
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["opencv_plot"]
+
+[tool.ruff.lint]
+extend-select = ["D", "I", "N", "RET", "UP"]
+
+[tool.ruff.lint.pydocstyle]
+convention = "pep257"
+
+[tool.ruff.lint.per-file-ignores]
+# Test names are the documentation here; a docstring would only repeat them.
+"tests/*" = ["D1"]