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.

README.md

@@ -35,13 +35,14 @@ wiring several shows them side by side, scaled to a common height. `bbox`, `text
 are overlays rather than panels — boxes are drawn on the `image` panel, in its own
 pixels, and the caption on the composite.
 
-### Runnable example
+### End-to-end test
 
-[`dataflow.yml`](dataflow.yml) is self-contained. [`examples/source.py`](examples/source.py)
+[`dataflow.yml`](dataflow.yml) is self-contained. [`tests/source.py`](tests/source.py)
 is a synthetic source node that emits an animated frame, a depth map of the same scene,
 the camera intrinsics, the box around the square moving in the frame, and a caption —
-every input this node draws — so the example needs no camera, no detector and no
-network:
+every input this node draws — so it needs no camera, no detector and no network. It is
+a test fixture rather than a worked example; the realistic wiring is the block at the
+top of this file. Run it to watch the node work:
 
 ```bash
 dora run dataflow.yml --uv
@@ -53,11 +54,8 @@ to quit.
 It builds the viewer from this checkout (`path:` + `build:`) rather than from a tag,
 which is what lets `dora validate` pick up the manifest described below.
 
-This example is also the end-to-end gate: CI runs the same dataflow headless on every
-push, which is what keeps it from rotting. It earns its place twice over — it is the
-only worked example of the *producing* side of these contracts, which is the fiddly
-half of wiring this node. The realistic wiring, against a real camera, is the block at
-the top of this file.
+CI runs the same dataflow headless on every push, which is what makes it a gate rather
+than a demo that rots.
 
 Set `SOURCE_FRAMES` to stop the source after a fixed number of frames instead of running
 until you quit; that plus `CI=true` is how the same dataflow runs headless as a test: