[rigtorp]

Add Linux ping-pong benchmarks for eventfd notifications and io_uring MSG_RING messages between two processes. Both report roundtrip and half-roundtrip latency; the io_uring benchmark warms up the rings, checks send completions and reply sequence numbers, and detects missing replies with a timeout.

The io_uring target is optional and requires liburing 2.2 or newer. Raise the CMake minimum to 3.10, fix signed/unsigned receive-loop warnings, and add basic build instructions. Simplify CI to a Linux/macOS matrix with push and PR checks, checkout v7, Linux liburing installation, and Release builds with C warnings treated as errors.

Validation:

• All 11 targets built locally with GCC and -Wall -Wextra -Wpedantic -Werror, using liburing 2.2.
• Configuration without liburing skips io_uring_lat and builds eventfd_lat successfully.
• Both new benchmarks passed small-count, invalid-argument, and 100,000-roundtrip checks.
• io_uring_lat returned an error after its peer was terminated instead of hanging.
• macOS validation is delegated to GitHub Actions.