feat(core/webrtc): expose Peer.Done() channel + AddICECandidate

Two small additions to support the M3 handler:

- Peer.Done() — read-only view of the existing 'done' channel,
  closed on Close(). Lets external indexes (Handler, admin API)
  await peer teardown without polling.
- Peer.AddICECandidate — passthrough so the WHEP PATCH handler
  can forward trickle-ICE candidates without reaching into the
  PeerConnection directly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/webrtc/peer.go

@@ -152,6 +152,12 @@ func (p *Peer) Answer() webrtc.SessionDescription { return p.answer }
 // ResourceID returns the stable resource id used in the WHEP Location header.
 func (p *Peer) ResourceID() string { return p.resourceID }
 
+// Done returns a channel that is closed when the Peer has been torn down
+// (either explicitly via Close, or because Pion observed an ICE
+// failure / disconnection). Consumers can range over it to drive
+// index cleanup without polling.
+func (p *Peer) Done() <-chan struct{} { return p.done }
+
 // Close tears down the peer connection and unsubscribes from each
 // source. Safe to call multiple times.
 func (p *Peer) Close() error {
@@ -257,6 +263,14 @@ func (f *PeerFactory) CreatePeerFromSources(ctx context.Context,
 	return p, nil
 }
 
+
+// AddICECandidate forwards a trickle-ICE candidate to the underlying
+// PeerConnection. Returns the underlying error if the candidate is
+// malformed or the connection has already been closed.
+func (p *Peer) AddICECandidate(c webrtc.ICECandidateInit) error {
+	return p.pc.AddICECandidate(c)
+}
+
 func newResourceID() string {
 	b := make([]byte, 8)
 	_, _ = rand.Read(b)