test: add cross-runtime integration test harness

[jiengup]

Summary

Related to #154

• Add a cross-runtime integration test harness for Tokio, Smol, and Compio.
• Introduce the #[runtime_test] procedural macro with runtime and capability filtering.
• Migrate mutex and semaphore integration tests away from Tokio-specific APIs.
• Add coverage for worker-task and thread-local task behavior.

Design Notes

The #[runtime_test] macro expands each annotated async function into an ordinary test for every matching runtime and injects a statically bound runtime alias into the test body. A shared runtime abstraction normalizes task spawning, thread-local execution, detached tasks, join errors, yielding, sleeping, and timeouts while preserving each executor’s scheduling model. The only(...) and require(...) arguments allow tests to target individual runtimes or declare required capabilities without introducing runtime-specific logic into the test itself.

Existing mutex and semaphore tests now use these normalized operations, with deterministic yielding replacing executor-specific timing where possible. Compio is constrained to the latest release series compatible with Rust 1.86, and its join behavior is adapted to the common harness interface. The semaphore coverage also uses the existing reduce_permits API to validate permit reduction and pending acquisition behavior consistently across all three runtimes.

Testing

• cargo +1.86.0 x test --no-capture
• cargo x test --no-capture
• cargo x check
• cargo x lint

All commands completed successfully.