New tests cover:
- background thread states after arena_reset
- background thread stats
- toggle background thread on and off during parallel stress
allocs/dallocs
- background thread fork behavior
Add background-thread fork unit test
test_fork_background_thread (in fork.c): with the background thread enabled,
fork and assert the child comes up with it disabled yet usable (re-enable +
alloc round-trips), while the parent keeps its threads. Placed in fork.c to
reuse its wait_for_child_exit helper and fork/WIN32 handling instead of
duplicating them in a separate file.