social: return registration cleanup from Subscribe - #20

Closing one broadcaster called social CloseContext, which unsubscribes the
shared RTA subscription and clears every handler on the client. That could
disable other Social().Subscribe users on a shared xsapi.Client.

Use the new go-xsapi social Client.Unsubscribe(ctx, handler) to remove only the
broadcaster's own handler; the RTA subscription is torn down only when its last
handler is gone. Bumps the go-xsapi replace to the commit that adds Unsubscribe.

Depends on HashimTheArab/go-xsapi#20; the replace pins that branch commit and
should be re-pinned to lunar once it merges.

* Accept friend requests reactively via social RTA subscription

Friend sync accepted incoming friend requests only on the poll interval, so a
new request waited up to one UpdateInterval to be accepted. Java MCXboxBroadcast
accepts them immediately off an RTA social subscription
(IncomingFriendRequestCountChanged -> acceptPendingFriendRequests).

Subscribe to each account's go-xsapi/v2 social RTA feed and feed its events into
the existing sync loop through a new FriendSyncer.Trigger channel, so an
incoming friend request (or any relationship change) runs an immediate sync pass
in the same goroutine that owns the rate-limit state. Events are coalesced
(non-blocking send to a size-1 channel). The subscription is best-effort: the
periodic syncer remains the backstop, the RTA connection re-subscribes after
transient drops, and a nil Trigger keeps a syncer purely periodic.

Closes #12.

* Undo social RTA subscription on broadcaster close

The social subscription created for reactive friend sync was never released.
b.ctx only scopes the subscribe handshake; once subscribed, go-xsapi keeps the
subscription active and appends a handler on each later Subscribe. A library
caller that reuses an external xsapi.Client across broadcaster restarts would
accumulate stale handlers, and events would keep dispatching after the syncer
stopped (Close only closes clients the broadcaster created itself).

Bind each subscription's lifetime to the broadcaster: a dedicated goroutine
unsubscribes once b.ctx is canceled, and Close waits on socialWg so the
subscriptions are undone before it returns. A reused client is left clean, so a
subsequent broadcaster re-subscribes without duplicating handlers.

* Guard against a closed friend-sync Trigger channel

A closed Trigger would receive forever, spinning Run through runSync and
bypassing UpdateInterval. Use a two-value receive and nil the channel out when
it is closed so the case stops firing.

* Release the social subscription per-handler instead of client-wide

Closing one broadcaster called social CloseContext, which unsubscribes the
shared RTA subscription and clears every handler on the client. That could
disable other Social().Subscribe users on a shared xsapi.Client.

Use the new go-xsapi social Client.Unsubscribe(ctx, handler) to remove only the
broadcaster's own handler; the RTA subscription is torn down only when its last
handler is gone. Bumps the go-xsapi replace to the commit that adds Unsubscribe.

Depends on HashimTheArab/go-xsapi#20; the replace pins that branch commit and
should be re-pinned to lunar once it merges.

* Use xsapi-owned social and presence clients

* Pin merged xsapi social subscription cleanup

* Bound reactive friend scans and honor read backoff

* Bound social RTA subscription setup