New src/foundation/subprocess.{h,c}: spawn a child, tail its output, and
classify how it ended — clean / exit-nonzero / crash / hang / killed — from
POSIX WIFSIGNALED/WTERMSIG and the Windows NTSTATUS exception exit codes
(0xC0000005 access violation, 0xC00000FD stack overflow). Adds an EINTR-safe
reap loop, a quiet-timeout that kills and reports a hung child (no new output
within the window), and partial-line-safe log tailing.
Generalized from the ad-hoc index spawn in src/ui/http_server.c so the
crash/hang supervisor can reuse one primitive across platforms. The exit-code
to outcome mapping lives in a pure cbm_proc_classify() so the Windows
crash-code path is unit-tested on every platform, not just an untested
Windows branch.
tests/test_subprocess.c: 13 tests — the classifier on all platforms plus real
POSIX spawn/reap of clean, non-zero, SIGSEGV-crash and hang children. Green
under ASan/UBSan.
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>
