Previously, the bootstrap handshake used a two-state machine
(WaitingForPorts / WaitingForReplyPort) to handle a race: the parent
registering transport ports and the child sending a bootstrap request
could arrive in either order, so whichever came first stored its half
and the second completed the handshake.
Eliminate the race by holding a mutex across spawn() and
register_child_transport(). Since the child cannot send a bootstrap
request before it exists, and the lock isn't released until its
transport is registered, handle_bootstrap_request() is guaranteed to
find the entry. This reduces the pending map to a simple pid-to-ports
lookup and collapses the two-variant state into two straightforward
branches: known child, or on-demand (non-child) caller like WebDriver.
e47f4cf90f