Acquires a shared subscription for subscribeFn. The first acquirer
establishes the transport; later acquirers with the same function
identity reuse it and immediately learn its current status. Returns a
release function -- the transport tears down only once every acquirer
has released.
Deduplicates concurrent calls to fn (by its own identity) with
canonically-equal params. Non-canonicalizable params (or a
non-canonicalizable-adjacent failure) simply never dedupe -- fn runs
fresh every time, never throwing and never silently colliding two
unrelated calls onto one shared promise.
Each caller's own signal only affects THEIR OWN wait -- aborting it
rejects this call's returned promise without affecting the shared
underlying execution or any other caller sharing it.
An isolated coordination domain for dedup and shared subscriptions -- see
createCoordinator/defaultCoordinator.