The overall `cmakeBuild` snippet is written only after the native build
tool returns, so interrupting `cmake --build` with Ctrl+C terminated CMake
before it was recorded and lost the build's delineation.
When instrumentation is active, install a scoped, async-signal-safe handler
(POSIX SIGINT; Windows CTRL_C/CTRL_BREAK) that just flags the interrupt. The
existing write then runs during unwind, records the interrupting signal in a
new `interruptSignal` field, skips the post-build index hook, and re-raises so
the exit status still reflects the signal.
The handler lives in its own translation unit, keeping the platform-divergent
signal code out of the main instrumentation implementation.
Issue: #27859
