[client] Refactor peer Conn to a single-owner event loop

Replace the mutex-guarded callback model of peer.Conn with a per-peer
event loop that exclusively owns all mutable connection state. External
callers and transport workers post typed events into a non-blocking,
coalescing mailbox instead of contending on conn.mu:

- offers/answers coalesce to the newest message, a new offer flushes
  queued candidates of the superseded session
- candidates are applied in arrival order from a bounded FIFO
- transport state changes are never dropped
- the blocking relay dial runs on a helper goroutine with a single dial
  in flight; signaling I/O (offer/answer sends) runs off the loop

conn.mu now only guards the open/close lifecycle. Close posts a close
event and waits for the loop teardown; the loop also tears down on
engine context cancellation and releases resources of unprocessed
events.

Delete the Handshaker listener machinery (Listen loop, unbuffered
drop-on-busy channels, AsyncOfferListener with its double-processing of
the first offer), the never-wired dispatcher package and the unused
ICEMonitor.ReconnectCh. Fix a goroutine leak in the WG watcher test
that raced with tests mutating the package-level check timing vars.

client/internal/peer/guard/ice_monitor.go

@@ -21,8 +21,6 @@ const (
 )
 
 type ICEMonitor struct {
-	ReconnectCh chan struct{}
-
 	iFaceDiscover stdnet.ExternalIFaceDiscover
 	iceConfig     icemaker.Config
 	tickerPeriod  time.Duration
@@ -34,7 +32,6 @@ type ICEMonitor struct {
 func NewICEMonitor(iFaceDiscover stdnet.ExternalIFaceDiscover, config icemaker.Config, period time.Duration) *ICEMonitor {
 	log.Debugf("prepare ICE monitor with period: %s", period)
 	cm := &ICEMonitor{
-		ReconnectCh:   make(chan struct{}, 1),
 		iFaceDiscover: iFaceDiscover,
 		iceConfig:     config,
 		tickerPeriod:  period,