feat: WebRTC data-plane framing, DTLS binding, and pc-leak fixes

- 1-byte-header fragmentation past the 64KB SCTP cap; empty-message and clean-EOF handling
- is_relayed() via selected candidate-pair stats for the direct/relayed flag
- IdPk.dtls_fingerprint + rendezvous webrtc SDP/IceCandidate proto fields
- fix pc leaks: Weak capture breaks the state-handler Arc self-cycle; close pc on new() error paths

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -68,7 +68,7 @@ rustls-pki-types = "1.11"
 rustls-native-certs = "0.8"
 webpki-roots = "1.0.4"
 async-recursion = "1.1"
-webrtc = { version = "0.14.0", optional = true }
+webrtc = { version = "0.13.0", optional = true }
 libloading = "0.8"
 
 [target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]
@@ -81,7 +81,7 @@ protobuf-codegen = { version = "3.7" }
 
 [dev-dependencies]
 clap = "4.5.51"
-webrtc = "0.14.0"
+webrtc = "0.13.0"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3", features = [