Detect tmpfs for a path with stat -f or findmnt -T, not /proc/mounts prefixes

Benchmarks that write to a scratch dir on tmpfs get near-free fsync: 64 x 1 MiB write+fsync took 18 ms on /dev/shm vs ~420 ms on ext4. Hand-rolled guards that pick the longest /proc/mounts mount point that prefixes the path miss symlinks: a symlink into /dev/shm matched '/' (overlay) until the path went through os.path.realpath(). stat -f -c %T DIR (prints tmpfs / ext2/ext3 / overlayfs) and findmnt -T DIR -no FSTYPE both resolve symlinks. Also, /tmp isn't always tmpfs: in Ubuntu containers it is often overlayfs with disk-like fsync cost, so measure on the filesystem you'll grade on.