SMBC: CI builds, regression tests and sanitizer checks

SMBC now has CI (#39). A GitHub Actions matrix builds and runs the headless suite on Ubuntu with gcc-13, gcc-14 and clang, on macOS, and under ASan/UBSan.

The port is c23, which surfaced toolchain grit: gcc-14's -Werror=parentheses, gcc-13 lacking -std=c23 (solved with -std=c2x fallback detection), mkstemp hidden behind POSIX feature gates on both glibc and Darwin under strict c23 (the replay tests now use a fixed build/test-replay.bin path plus remove()), and glibc fortify making system()'s return value mandatory.