fix: align certificate types and close gathering state

[mertushka]

Summary

Align certificate generation types with the supported data-channel-first runtime behavior, and reset iceGatheringState after RTCPeerConnection.close().

The runtime rejects string-form certificate algorithms, so the TypeScript declaration now only advertises the supported object form. Closing a peer now clears the cached ICE gathering state back to new, matching the no-transports state exposed after close.

Verification

• npm run check
• npm run native:check - not applicable; no native source change
• npm run build - not applicable; no native or build-system change
• npm test
• npm run api:check
• npm run types:check
• npm run e2e:chrome - not applicable; no Chrome/browser interop path change
• npm run wpt:selection:check
• npm run wpt:smoke
• npm run wpt:smoke:check
• npm run wpt:test / npm run wpt:check:strict - not run locally; full WPT is reserved for conformance validation

WPT impact

wpt-manifest.json is unchanged. docs/divergences.md now documents the object-form-only certificate generation behavior.

Browser interoperability

No Chrome E2E behavior or coverage changes.

Notes

No libdatachannel pin, WPT pin, native lifetime, callback-threading, workflow, release, permission, or publication impact.