* [client] fix iOS route-update reordering that black-holed IPv6 on exit-node disable
On iOS the route notifier delivered each prefix update from its own
fire-and-forget goroutine (notify -> `go func`), so Go provided no ordering
guarantee between consecutive updates. It also read currentPrefixes inside
that goroutine without holding the lock, racing the next OnNewPrefixes write.
On exit-node disable the core removes the default routes as two separate
prefix updates (0.0.0.0/0, then the synthesized ::/0). When the two
goroutines were reordered, the stale snapshot still containing ::/0 was
delivered last and clobbered the correct default-free one. iOS then kept the
::/0 default route on the tunnel with no exit node to carry it, black-holing
all IPv6 traffic while IPv4 recovered correctly.
Fix: deliver updates through a single worker goroutine fed by a buffered
channel, preserving production order, and snapshot the joined prefix string
under the mutex so it can't race a concurrent update. Buffered so producers
(which run under the route manager lock) don't block on the listener callback.
* [client] close iOS notifier delivery goroutine on Stop, unbounded queue
The delivery goroutine was never stopped, leaking on every engine
restart. Add Notifier.Close, called from the route manager Stop after
routing cleanup.
Replace the buffered update channel with a cond-driven linked-list
queue so route-update producers (running under the route manager lock)
never block when the listener callback is slow.
extraInitialRoutes() was meant to preserve only the fake IP route
(240.0.0.0/8) across TUN rebuilds, but it re-injected any initial
route missing from the current set. When the management server
advertised exit node routes (0.0.0.0/0) that were later filtered
by the route selector, extraInitialRoutes() re-added them, causing
the Android VPN to capture all traffic with no peer to handle it.
Store the fake IP route explicitly and append only that in notify(),
removing the overly broad initial route diffing.
