CI: build all targets in both standalone and superproject modes

[sgerbino]

Summary

CI should build all target categories — library, tests, examples, and benchmarks — and do so in both standalone and Boost superproject configurations.

Current state

Some target categories may not be exercised in CI, meaning build regressions can go unnoticed until downstream consumers hit them.

Proposed changes

• Build and link all example targets in CI
• Build and link all benchmark targets in CI
• Build and run all test targets in CI
• Verify all of the above in standalone mode (library consumed directly)
• Verify all of the above in Boost superproject mode (boost/libs/capy within the full tree)
• Cover at minimum: GCC, Clang, MSVC across the supported OS matrix

[sgerbino]

This issue may be a good opportunity to cleanup CMakeLists.txt. It should be treated like code -- because it is. I started work cleaning up CMakeLists.txt for corosio and it can be used as reference. There are at least two build scenarios we need to support.

1. Standalone (not within a boost superproject). Users should be able to add_subdirectory() OR FetchContent to use the library.
2. Within the boost superproject (which should give us access to dependencies like Asio for examples)