lib: gather shared helper functions

lib: gather shared helper functions

Extract process-neutral helpers from bees-util.cc, bees-roots.cc, and
bees-trace.cc into a new bees-lib.cc so test binaries can link against
them without pulling in bees.cc (which contains main()).

Functions moved:
  pretty(), format_time(), exception_check(),
  bees_openat(), bees_openat2(),
  bees_readahead_check(), bees_readahead_nolock(),
  bees_readahead_pair(), bees_readahead(), bees_unreadahead(),
  bees_throttle() and bees_throttle_factor.

Assisted-by: Claude-Code:claude-opus-4-7
Assisted-by: Claude-Code:claude-sonnet-4-6
Signed-off-by: Zygo Blaxell <bees@furryterror.org>