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.

.gitignore

@@ -0,0 +1,11 @@
+__pycache__/
+*.py[cod]
+.venv/
+.pytest_cache/
+.ruff_cache/
+dist/
+
+# dora build working directories
+.dora/
+git/
+out/