feat: add typed libdatachannel extensions

[mertushka]

Purpose

Expose a narrow typed nonstandard surface for libdatachannel capabilities while preserving the existing W3C-facing RTCPeerConnection, RTCDataChannel, and setLocalDescription() behavior.

Changes

• add a GC-safe nonstandard.IceUdpMuxListener backed by rtc::IceUdpMuxListener
• add pre-construction peer options for UDP mux, fingerprint verification, and maximum message size
• add explicit initial local ICE credentials through rtc::LocalDescriptionInit
• expose the observed remote certificate fingerprint
• add validated PEM certificate/private-key import without exposing key material on RTCCertificate
• update TypeScript declarations, API/native checks, focused tests, and technical documentation

Behavior impact

The standard WebRTC facade remains unchanged unless a nonstandard extension is explicitly used. Fingerprint verification stays enabled by default. Explicit ICE credentials preserve browser-shaped local description, signaling, gathering, and event timing until standard setLocalDescription() is called.

Native impact

The addon remains Node-API-only. UDP mux callbacks are delivered through a Napi::ThreadSafeFunction; repeated close, garbage collection, late queued callbacks, and environment shutdown are handled explicitly.

WPT and browser impact

No WPT selection or expected result classification changes. Local validation passed:

• npm run check
• npm run native:check
• npm run build
• npm test (63/63)
• npm run api:check
• npm run types:check
• npm run pack:check
• npm run wpt:selection:check
• npm run wpt:smoke and npm run wpt:smoke:check (4/4, no retries)
• targeted offer/answer/local-description/certificate WPT (12/12, no retries)
• npm run e2e:chrome (12/12 scenarios)

The full selected WPT suite is delegated to the branch Conformance workflow.