[META] WebTransport support

**NOTE: We will not accept a PR regarding this issue right now, due to the volume of code it modifies. We are satisfied with our WebSockets + WebCodecs implementation as of now, which is compatible with all major reverse proxies, web browsers, and localhost environments.**

Can be AI-driven: Yes, as long as all cases are tested carefully in real-life by people as well as automated tests. However, **expert guidance is required due to working with protocol intrinsics**.
Expected to be workable with: Fable 5, GPT-5.6 Sol, or equivalent (max thinking, requires expert guidance, especially with Opus 5, Kimi-K3, or under).

However, I also have doubts about WebTransport's practical usefulness as of 2026, because it requires complex procedures in local environments, such as HTTP not being supported at all, and self-signed certificates requiring specialized digest delivery mechanisms.

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https://web.dev/i18n/en/webtransport/
https://www.w3.org/TR/webtransport/
https://web.dev/webcodecs/
https://www.w3.org/TR/webaudio/
https://web.dev/media-mse-basics/

WebTransport is a promising new web protocol based on HTTP3. This combined with Media Source Extensions, WebCodecs, and Web Audio API, will enable Selkies to provide equal performance compared to the native UDP protocols used by many high-performance remote desktop protocols such as [Parsec](https://parsec.app). A TURN server will no longer be required to be used, as WebTransport establishes a client-server connection.

~~This will enable more codecs not restrained to WebRTC specifications, such as H.265 HEVC and AV1, as well as **bringing the performance equal to existing desktop client implementations such as Sunshine or Parsec.**~~

~~However, a WebTransport plugin for [GStreamer](https://gstreamer.freedesktop.org) is not yet ready.~~ Look below.

No repository file, test, or entry point is identified. Start by reading the linked WebTransport, WebCodecs, Web Audio, and Media Source specifications, then clarify the implementation scope and acceptance criteria with maintainers; the issue states that PRs are not currently accepted and that expert guidance is required.

Written by the indexing model from the issue text.