add dora-webcam node

Captures frames from a V4L2 camera and publishes them as std/media/v1/Image:
an Arrow struct of width, height, encoding and data, so a receiver needs
nothing from the message metadata. For a compressed encoding the recorded
dimensions are the image's, not the length of the payload.

Based on opencv-video-capture from dora-hub, with three changes. The output is
the typed image struct rather than a flat UInt8 buffer plus metadata. Camera
selection by id reads /dev/v4l/by-id/ only; the upstream macOS system_profiler
and Windows PowerShell enumeration is not carried. CAPTURE_FRAMES bounds a run
so a test ends on its own, in place of upstream's ten-second CI cutoff -- a
timeout would have made the CI step exit non-zero.

CI=true keeps a failed capture from raising, publishing a labelled black frame
instead, which is what lets dataflow.yml run on a machine with no camera.

Two dataflows, run one at a time, as dora has no cross-dataflow wiring:
dataflow.yml is the camera alone and the CI gate; dataflow_view.yml wires it
into dora-image-view and is not run by CI, since it fetches that node over the
network and needs a tag to exist there.

pyproject.toml

@@ -0,0 +1,44 @@
+[project]
+name = "dora-webcam"
+version = "0.1.0"
+description = "Dora node that captures frames from a webcam and publishes them as std/media/v1/Image"
+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.
+dependencies = [
+  "dora-rs >= 1.0.1",
+  "numpy >= 2.0",
+  "opencv-python >= 4.9",
+  "pyarrow >= 17.0",
+]
+
+[project.scripts]
+dora-webcam = "dora_webcam.main:main"
+
+[dependency-groups]
+dev = ["pymarkdownlnt >= 0.9.30", "pytest >= 8.1", "ruff >= 0.9"]
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["dora_webcam"]
+
+[tool.ruff.lint]
+extend-select = ["D", "I", "N", "RET", "UP"]
+
+[tool.ruff.lint.pydocstyle]
+convention = "numpy"
+
+[tool.ruff.lint.per-file-ignores]
+# Test names are the documentation here; a docstring would only repeat them.
+"tests/*" = ["D1"]
+
+[tool.pymarkdown]
+# The prose here wraps near ruff's 88; 100 leaves room for a long link or table
+# row without inviting unwrapped paragraphs.
+plugins.md013.line_length = 100