identityOf method
The OS identity of the named LIVE session — its leader pid and (when resolution succeeded at spawn) its pgid — or null when the session is unknown, already terminal, or its spawn has not yet stamped a pid.
The synchronous surface an acquire that lost the SessionStarted race
resolves a handle from (tg-090/D5): for a session that already existed,
SessionStarted fired before this incarnation subscribed and never
re-fires — never wait for it; query this (and terminalOf, which covers
the already-dead case) instead.
Implementation
@override
({int pid, int? pgid})? identityOf(String name) =>
_live.contains(name) ? identities[name] : null;