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.

dora-node.yml

@@ -0,0 +1,84 @@
+# 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 a camera nor
+# OpenCV.
+apiVersion: 1
+name: dora-webcam
+namespace: liria
+description: Captures frames from a webcam and publishes them as std/media/v1/Image
+categories: [sensor]
+keywords: [opencv, camera, webcam, video, capture, image, v4l2]
+
+runtime: python
+entrypoint: dora-webcam
+# `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:
+  tick:
+    required: true
+    description: >-
+      Reads and publishes one frame. Wire to dora/timer/millis/N. The node stops
+      when this input closes.
+
+outputs:
+  image:
+    type: std/media/v1/Image
+    description: >-
+      Captured frame as an Arrow struct {width, height, encoding, data}. For a
+      compressed encoding, width and height are the image's dimensions rather
+      than the length of data, so a receiver can size a buffer without decoding
+      first.
+
+env:
+  CAMERA_ID:
+    description: >-
+      Substring of an entry in /dev/v4l/by-id/. Stable across reboots and across
+      which USB port the camera is in, unlike an index, so it takes precedence
+      over CAPTURE_PATH. Linux only.
+  CAPTURE_PATH:
+    default: "0"
+    description: Device path such as /dev/video1, or an index. A numeric string is read as an index.
+  IMAGE_WIDTH:
+    type: int
+    description: Requested capture width; frames are also resized to it. Defaults to the camera's native width.
+  IMAGE_HEIGHT:
+    type: int
+    description: Requested capture height; frames are also resized to it. Defaults to the camera's native height.
+  ENCODING:
+    default: bgr8
+    description: One of bgr8, rgb8, yuv420, jpeg/jpg/jpe, png, bmp, webp.
+  JPEG_QUALITY:
+    type: int
+    default: 95
+    description: Quality 0-100, used only by the JPEG encodings.
+  FLIP:
+    description: Flip the frame. One of VERTICAL, HORIZONTAL or BOTH. Unset means no flip.
+  CAPTURE_FRAMES:
+    type: int
+    default: 0
+    description: >-
+      Stop after this many frames. 0, the default, runs until the tick input
+      closes. A positive count is what lets a test run the dataflow to
+      completion rather than killing it on a timeout.
+  CI:
+    type: bool
+    description: >-
+      When true, a failed read publishes a labelled black frame instead of
+      raising, which is what lets the dataflow run on a machine with no camera.
+
+example: |
+  - id: camera
+    git: ssh://git@gitea.liria.cl:5001/liria/dora-webcam.git
+    tag: v0.1.0
+    build: pip install .
+    path: dora-webcam
+    inputs:
+      tick: dora/timer/millis/33
+    outputs:
+      - image