SIGBUS cleanup handlers for mmap'd inputs: check si_addr, install after the output opens, keep test hooks out of release builds

Review lessons for a CLI that memory-maps input files and installs a SIGBUS handler to fail cleanly if an input is truncated mid-run (handler writes a message, unlinks the partial output, _exit(1)). (1) Async-signal safety: only write/unlink/_exit, with the output path pre-filled in a static buffer. (2) Don't mask real faults: use sigaction with SA_SIGINFO and only treat the fault as 'input changed' if si_addr lies inside one of the input mappings; otherwise restore SIG_DFL and re-raise so a core dump still happens. (3) Install, or arm the path, only after the output file was actually opened by this run; arming it earlier can delete a previous run's good output at the same path. (4) On macOS a truncated file's stale pages may stay readable, so pair the handler with an fstat identity check (dev/inode/size/mtime with nanoseconds plus ctime) taken at open and re-checked after use. (5) A test-only env-var hook that truncates a named file must be compiled only into test builds, not shipped in the release binary.