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.

dataflow.yml

@@ -21,12 +21,9 @@ nodes:
       - image
       - bbox
       - text
-    output_types:
-      image: std/core/v1/UInt8
-      text: std/core/v1/String
 
-  - id: plot
-    path: dora-plot
+  - id: view
+    path: dora-image-view
     build: pip install .
     inputs:
       image: source/image