deps(transport-webrtc): replace node-datachannel

[mertushka]

Description

Replaces node-datachannel with @mertushka/webrtc-node in @libp2p/webrtc.

@mertushka/webrtc-node is a Node.js WebRTC binding that exposes a W3C-compatible WebRTC API surface instead of a package-specific polyfill surface. Version 0.2.0 passes 620 strict Web Platform Tests for WebRTC API behavior and provides the standard objects @libp2p/webrtc already uses in browsers, including RTCPeerConnection, RTCDataChannel, RTCSessionDescription, RTCIceCandidate, RTCCertificate, and standard EventTarget behavior.

This is useful here because the existing Node path had to wrap node-datachannel APIs to inject ICE credentials, map ICE servers, import certificates, work around stale data channel state, and force native peer connection cleanup. With @mertushka/webrtc-node, the Node path can use the standard WebRTC-shaped APIs directly, with explicit nonstandard hooks only where WebRTC Direct needs Node-specific control over ICE credentials, certificate import, UDP muxing, fingerprint verification, and remote fingerprint access.

This removes the node-datachannel-specific compatibility layer and related runtime workarounds now that the replacement exposes the required standard WebRTC APIs plus the nonstandard APIs used by the WebRTC Direct path.

Changes include:

• replace the node-datachannel dependency with @mertushka/webrtc-node
• wire the new WebRTC implementation through the Node WebRTC entrypoints
• keep WebRTC Direct support using nonstandard APIs:
  • configurePeerConnection
  • importCertificate
  • setLocalIceCredentials
  • getRemoteFingerprint
  • IceUdpMuxListener
• remove node-datachannel-specific peer connection wrappers
• remove stale readyState/send error workarounds
• remove node-datachannel GC/closed-state workarounds

Related open PRs

Potentially Fixes/Closes:

• feat: evaluate @roamhq/wrtc as alternative to node-datachannel #3356
• fix(transport-webrtc): detect terminal state at construction time #3459
• fix(transport-webrtc): fix EOF race in readCandidatesUntilConnected #3460

Likely requires a rebase:

• feat: webrtc-direct-v2 #3480

Verification

• npm run dep-check -w @libp2p/webrtc
• npm run build -w @libp2p/webrtc
• npm run test:node -w @libp2p/webrtc -- --exit
• npm run test:chrome -w @libp2p/webrtc