VFS: harden crash path so FUSE bad_alloc no longer deadlocks system

Wrap every FUSE callback in a noexcept thunk that catches std::bad_alloc
(→ ENOMEM) and other exceptions (→ EIO) so they never unwind into
libfuse3 (C, no unwind support). Stops std::terminate from orphaning
the mount.

Crash handler: replace raise(sig) with _exit(128+sig) — raise can hang
when the signal is masked on libfuse worker threads. Try direct
umount2(MNT_DETACH) before fork+exec fusermount3 -uz, and double-fork
the fallback so we don't waitpid on a stuck child. Switch signal() to
sigaction(SA_RESETHAND), add SIGBUS, install std::set_terminate.

Cap readdir/readdirplus kernel-supplied size at 1 MiB to defend against
oversized vector(size) allocations.

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