feat: implement §9 file transfer over dedicated binary DataChannels

File chunks travel over a per-transfer "f:<xid>" WebRTC DataChannel —
direct DTLS-encrypted P2P, the anchor never sees file data. Once the
initial handshake is done the anchor can disappear and transfers continue.

Key design choices:
- Receiver sends "ok" on the file DC before sender streams, eliminating
  a race where tiny files could be fully sent/closed before the receiver's
  OnMessage handler is registered (open-race §6 analogue for data DCs).
- Auto-accept: receiver accepts every incoming offer immediately.
- Download dir: per-network at <data-dir>/downloads-<netid>/.
- Backpressure: bufferedAmountLowThreshold to avoid overwhelming the DC.
- SHA-256 verified on receive; mismatches emit EvtError and discard temp file.
- IPC: send_file {peer_id, path} → offers the named file from share dir.
- EvtFileComplete {transfer_id, path} emitted on success.

IPC command: {"type":"send_file","peer_id":"<hex>","path":"<filename>"}

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/anchor/client.go

@@ -230,6 +230,14 @@ func offer(peerID proto.PeerID, id *crypto.Identity, m *mesh.Mesh, s *sender) (*
 	mesh.WireDataChannel(dc, pc, peerID, id, m)
 	mesh.WireCandidateTrickle(pc, peerID, s)
 
+	// Handle file DataChannels opened by the remote peer (they are the file sender).
+	pc.OnDataChannel(func(dc *webrtc.DataChannel) {
+		if strings.HasPrefix(dc.Label(), "f:") {
+			xid := strings.TrimPrefix(dc.Label(), "f:")
+			m.HandleInboundFileDC(dc, xid, peerID)
+		}
+	})
+
 	sdpOffer, err := pc.CreateOffer(nil)
 	if err != nil {
 		pc.Close()
@@ -248,8 +256,12 @@ func answer(payload proto.SignalingPayload, fromID proto.PeerID, id *crypto.Iden
 		return nil, err
 	}
 	pc.OnDataChannel(func(dc *webrtc.DataChannel) {
-		if dc.Label() == "yaw" {
+		switch {
+		case dc.Label() == "yaw":
 			mesh.WireDataChannel(dc, pc, fromID, id, m)
+		case strings.HasPrefix(dc.Label(), "f:"):
+			xid := strings.TrimPrefix(dc.Label(), "f:")
+			m.HandleInboundFileDC(dc, xid, fromID)
 		}
 	})
 	mesh.WireCandidateTrickle(pc, fromID, s)