Prototype a continuous integration workflow

This workflow is too spendthrift for deployment: every job sets up Rust,
and the `build` job also installs Trunk, which takes even more
resources. The workflow serves well, however, as a proof of concept.
Introducing a syntax error into `src/main.rs` causes the `build` and
`test` jobs to fail, but allows `run-examples` to succeed. Similarly,
introducing a syntax error into `examples/point-on-sphere.rs` causes
`run-examples` to fail, but allows the other jobs to succeed.

.forgejo/run-examples/action.yaml

@@ -0,0 +1,12 @@
+# run all Cargo examples, as described here:
+#
+#   Karol Kuczmarski. "Add examples to your Rust libraries"
+#   http://xion.io/post/code/rust-examples.html
+#
+runs:
+  using: "composite"
+  steps:
+    - run: cd app-proto; cargo run --example irisawa-hexlet
+    - run: cd app-proto; cargo run --example three-spheres
+    - run: cd app-proto; cargo run --example point-on-sphere
+    - run: cd app-proto; cargo run --example kaleidocycle