fix strict-type validation by giving the example source a manifest

`dora validate` matches a node manifest to a `path:` node by entrypoint. With
no manifest beside examples/source.py the root dora-node.yml was tried against
it, did not match its entrypoint, and warned -- which `strict_types: true`
turns into a non-zero exit, failing CI.

Declaring the source's output types in the new manifest rather than as
`output_types:` in dataflow.yml is also what makes them checked: an inline
annotation shadows the manifest, so the manifest would be injected but never
compared against the viewer's.

Verified with dora-cli 1.0.1: validate passes with both contracts injected, and
`CI=true SOURCE_FRAMES=30 dora run dataflow.yml --uv` runs both nodes to a
clean exit.

examples/dora-node.yml

@@ -0,0 +1,52 @@
+# Manifest for the example source node.
+#
+# Its reason to exist is as much mechanical as documentary: `dora validate`
+# matches a node manifest to a `path:` node by entrypoint, and without a
+# manifest here the repository-root dora-node.yml -- whose entrypoint is
+# `dora-image-view` -- gets tried against `examples/source.py`, does not match,
+# and warns. Under `strict_types: true` that warning fails the run.
+#
+# Declaring the types here rather than as `output_types:` in dataflow.yml is
+# what makes them checked: an inline annotation shadows the manifest, so the
+# manifest would be injected but never compared against dora-image-view's.
+apiVersion: 1
+name: source
+namespace: liria
+description: Synthetic image, bbox and text source for the dora-image-view example
+categories: [debug]
+keywords: [example, test, synthetic]
+
+runtime: python
+entrypoint: source.py
+build: pip install .
+dora: ">=1.0"
+
+inputs:
+  tick:
+    required: true
+    description: Timer tick; one frame is emitted per tick.
+
+outputs:
+  image:
+    type: std/core/v1/UInt8
+    description: >-
+      Animated bgr8 frame as a flat UInt8 buffer, with metadata
+      {width, height, encoding}.
+  bbox:
+    description: >-
+      The box around the moving square: an Arrow struct {bbox, conf, labels}
+      with metadata {format: xyxy}. Untyped to match the dora-image-view input.
+  text:
+    type: std/core/v1/String
+    description: A fixed caption.
+
+env:
+  SOURCE_WIDTH:
+    type: int
+    description: Frame width in pixels. Defaults to 640.
+  SOURCE_HEIGHT:
+    type: int
+    description: Frame height in pixels. Defaults to 480.
+  SOURCE_FRAMES:
+    type: int
+    description: Stop after this many frames. 0, the default, runs until the viewer quits.