CI: build all targets (library, tests, examples, benchmarks) standalone and in superproject

[sgerbino]

Summary

Ensure CI builds all CMake targets — library, tests, examples, and benchmarks — in both standalone and Boost superproject modes.

Current state

• The primary CI matrix builds within the Boost superproject via B2 and CMake.
• There is no CI job that builds corosio as a fully standalone project (outside the Boost tree entirely), where Capy is fetched via FetchContent.
• Benchmark and example targets may be skipped in some configurations.

Desired state

1. All target categories built in CI:

   • boost_corosio (core library)
   • boost_corosio_openssl / boost_corosio_wolfssl (TLS providers, where available)
   • Unit tests (boost_corosio_tests)
   • Examples (client, echo-server, nslookup, https-client)
   • Benchmarks (corosio_bench)

2. Both build modes exercised:

   • Superproject: Build within the Boost tree (B2 and/or CMake), as currently done.
   • Standalone: A dedicated CI job that builds corosio entirely outside the Boost superproject — corosio is the CMake root and Capy is fetched via FetchContent. Build with BOOST_COROSIO_BUILD_TESTS=ON, BOOST_COROSIO_BUILD_EXAMPLES=ON, and BOOST_COROSIO_BUILD_PERF=ON.

3. Explicit verification that each target compiles successfully in both modes.

Suggested approach

• Add standalone CI matrix entries to ci.yml that configure and build corosio as a standalone root project with all build options enabled.
• Ensure existing superproject jobs also enable examples and benchmarks (or add a dedicated job that does).
• Consider a CI step that lists built targets to catch regressions where a target silently stops being built.