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_webcam/image.py

@@ -0,0 +1,48 @@
+"""Packing a frame as std/media/v1/Image.
+
+The struct is respelled here rather than imported from the viewer: node
+repositories share no source package, so the manifest and this schema are the
+whole contract between them.
+"""
+
+import numpy as np
+import pyarrow as pa
+
+IMAGE_TYPE = pa.struct([
+    ("width", pa.uint32()),
+    ("height", pa.uint32()),
+    ("encoding", pa.utf8()),
+    ("data", pa.large_binary()),
+])
+
+
+def pack(frame: np.ndarray, encoding: str, width: int, height: int) -> pa.Array:
+    """Pack encoded frame bytes as a single-row std/media/v1/Image array.
+
+    Parameters
+    ----------
+    frame : np.ndarray
+        The bytes to send. Either a raw pixel buffer or the output of an
+        encoder; either way only its bytes are read.
+    encoding : str
+        Name of the encoding the bytes are in, e.g. ``bgr8`` or ``jpeg``.
+    width, height : int
+        Dimensions of the *image*, which for a compressed encoding are not the
+        shape of ``frame``. They are recorded so a receiver can size a buffer
+        without decoding first.
+
+    Returns
+    -------
+    pa.Array
+        One-row struct array matching ``IMAGE_TYPE``.
+
+    """
+    return pa.array(
+        [{
+            "width": width,
+            "height": height,
+            "encoding": encoding,
+            "data": frame.tobytes(),
+        }],
+        type=IMAGE_TYPE,
+    )