refactor: type the example source inline instead of shipping a manifest

A dora-node.yml describes exactly one node and is the unit `dora hub publish`
operates on, which expects it beside pyproject.toml. This repository ships one
node, so it should carry one manifest. examples/dora-node.yml described a test
fixture that nobody installs and that has no package of its own -- a manifest
for something that is not a node.

The source's outputs are now annotated with `output_types:` in the dataflow,
which is the documented way to type an output without a manifest: "only
annotated outputs are type-checked". It runs as `path: python` + `args:`
because `dora validate` matches a manifest to a `path:` node by entrypoint, so
a path node would be tried against the root manifest, fail to match
`dora-image-view`, and warn -- fatal under strict_types. Through the
interpreter there is nothing to match.

The type gate is unchanged in strength: the viewer's contract is still
injected and compared against the source's annotations, and a deliberate
Float32 -> String swap still fails validate.

The cost is that the fixture no longer gets a managed venv of its own and runs
against the project environment. That applies to the fixture only; the shipped
node still uses path: + build:, which is what a consumer exercises.

This supersedes the approach in "fix strict-type validation by giving the
example source a manifest" and the doc commit that cited the
manifest-next-to-a-path-node rule as justification.

README.md

@@ -49,7 +49,7 @@ dora run dataflow.yml --uv
 A window opens with a square bouncing across a gradient, boxed and labelled. Press `q`
 to quit.
 
-It builds both nodes from this checkout (`path:` + `build:`) rather than from a tag,
+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.
 
 Set `SOURCE_FRAMES` to stop the source after a fixed number of frames instead of running
@@ -66,16 +66,19 @@ CI=true SOURCE_FRAMES=30 dora run dataflow.yml --uv
 [`dora-node.yml`](dora-node.yml) declares the node's inputs with type URNs. No source
 package is shared between node repositories, so it is the only contract a consumer has,
 and it is what makes a wiring mistake fail at build time instead of at runtime. The
-example source has a manifest of its own, so `dora validate` checks the two against
-each other:
+example source is typed inline in the dataflow with `output_types:`, so `dora validate`
+checks the two against each other:
 
 ```
 $ dora validate dataflow.yml
-  node "source": injected 2 contract type(s) from .../examples/dora-node.yml
-  node "view": injected 2 contract type(s) from .../dora-node.yml
+  node "view": injected 4 contract type(s) from .../dora-node.yml
 All type annotations OK.
 ```
 
+There is one manifest because this repository ships one node. The example source is a
+test fixture rather than something anyone installs, so it has none, and runs through
+the interpreter to keep `dora validate` from matching the viewer's manifest against it.
+
 `image` is `std/core/v1/UInt8`, `depth` and `camera_info` are `std/core/v1/Float32` and
 `text` is `std/core/v1/String`; the payloads are raw buffers described by their
 metadata, so the URN names the element type rather than a struct. `bbox` is deliberately