move the synthetic source into tests

It is a test fixture, so it lives in tests/. It was only ever an example by
placement: CI runs it as the end-to-end gate, SOURCE_FRAMES exists so CI can
bound it, and it publishes depth and camera_info because those inputs are typed
and strict validation fails on a typed input the dataflow does not wire -- none
of which is pedagogy. The worked example of wiring this node is the README's
usage block, against a real camera.

It stays a dora node rather than a pytest case because what it exercises is the
wiring and the event loop, which only a running dataflow covers. pytest does
not collect it: the filename does not start with `test_`.

dataflow.yml stays at the repository root rather than moving beside it. A
node's working directory is the directory of the dataflow file, and both nodes
build with `pip install .`, which needs pyproject.toml in that directory.

dataflow.yml

@@ -1,6 +1,11 @@
-# The repository's own example, and its end-to-end test: a synthetic source
-# feeding the viewer. Everything it needs is in this checkout -- no camera, no
-# detector, no network -- so it is also what CI runs.
+# The end-to-end test: a synthetic source feeding the viewer. Everything it
+# needs is in this checkout -- no camera, no detector, no network -- which is
+# what lets CI run it rather than only validate it.
+#
+# It lives at the repository root rather than beside tests/source.py because a
+# node's working directory is the directory of the dataflow file, and both
+# nodes build with `pip install .`, which needs pyproject.toml in that
+# directory.
 #
 # Both nodes are built from the checkout with `path:` + `build:`, which is what
 # lets `dora validate` pick up the typed contract in dora-node.yml. A consumer
@@ -11,7 +16,7 @@ strict_types: true
 
 nodes:
   - id: source
-    # Run through the interpreter rather than as `path: examples/source.py`.
+    # Run through the interpreter rather than as `path: tests/source.py`.
     #
     # A dora-node.yml describes exactly one node and is what gets published
     # (`dora hub publish` expects it beside pyproject.toml), and this
@@ -25,7 +30,7 @@ nodes:
     # interpreter leaves nothing for it to match, and `output_types` below is
     # the documented way to type an output without one.
     path: python
-    args: examples/source.py
+    args: tests/source.py
     # The source needs dora-rs, numpy and pyarrow; installing the project gives
     # it exactly the versions pinned in uv.lock.
     build: pip install .