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.

examples/dora-node.yml

@@ -1,66 +0,0 @@
-# Manifest for the example source node.
-#
-# A manifest describes exactly one node, so a second node in this repository
-# gets a second manifest. This is the documented way to type a node you have
-# not published: "a dora-node.yml sitting next to a plain `path:` node is
-# picked up automatically" -- https://dora-rs.ai/dora/hub/node-manifest
-#
-# It is not optional here. `dora validate` matches a manifest to a `path:` node
-# by entrypoint; with no manifest beside examples/source.py the repository-root
-# one is tried against it, does not match its `dora-image-view` entrypoint, and
-# warns -- which `strict_types: true` turns into a failure.
-#
-# 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, depth, camera_info, 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.
-  camera_info:
-    type: std/core/v1/Float32
-    description: >-
-      [fx, fy, cx, cy] for the frames above. Published once, on the first tick,
-      the way a real camera publishes it once per session.
-  depth:
-    type: std/core/v1/Float32
-    description: >-
-      Depth of the same scene in millimetres, [height, width]: a flat wall with
-      the square standing in front of it.
-
-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.