feat: Add experimental rewrite of go2rtc live provider (MSE/WebRTC/MP4/MJPEG) (#2580)

- Closes #2556
- Closes #2450

**Key intended features:**
 - go2rtc compatible
- 100% test coverage to significantly improve ability to test, maintain
and work around browser weirdnesses (e.g. Safari).
 - Written from the ground up in the style of the rest of the project.

**To use:**
 - Change `live_provider` from `go2rtc` to `go2rtc-experimental`.

tests/components-lib/live/providers/go2rtc-experimental/offscreen-video.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, it, vi } from 'vitest';
+
+import { OffscreenVideo } from '../../../../../src/components-lib/live/providers/go2rtc-experimental/offscreen-video';
+import { FakeMediaStream, FakeMediaStreamTrack } from './test-utils';
+
+// @vitest-environment jsdom
+describe('OffscreenVideo', () => {
+  it('should create the video from the factory on get', () => {
+    const video = document.createElement('video');
+    const offscreen = new OffscreenVideo(() => video);
+
+    expect(offscreen.get()).toBe(video);
+  });
+
+  it('should reuse the same video across repeated get', () => {
+    const create = vi.fn(() => document.createElement('video'));
+    const offscreen = new OffscreenVideo(create);
+
+    expect(offscreen.get()).toBe(offscreen.get());
+    expect(create).toBeCalledTimes(1);
+  });
+
+  it('should create a video with the default factory when none is injected', () => {
+    const offscreen = new OffscreenVideo();
+
+    expect(offscreen.get()).toBeInstanceOf(HTMLVideoElement);
+  });
+
+  it('should detach src and srcObject on clear', () => {
+    const video = document.createElement('video');
+    const offscreen = new OffscreenVideo(() => video);
+    offscreen.get();
+    video.src = 'data:video/mp4;base64,AAAA';
+    video.srcObject = new FakeMediaStream([
+      new FakeMediaStreamTrack('video'),
+    ]).asMediaStream();
+
+    offscreen.clear();
+
+    expect(video.hasAttribute('src')).toBe(false);
+    expect(video.srcObject).toBeNull();
+  });
+
+  it('should create a fresh video after clear', () => {
+    const create = vi.fn(() => document.createElement('video'));
+    const offscreen = new OffscreenVideo(create);
+    offscreen.get();
+    offscreen.clear();
+    offscreen.get();
+
+    expect(create).toBeCalledTimes(2);
+  });
+
+  it('should tolerate clear when no video is held', () => {
+    const offscreen = new OffscreenVideo(() => document.createElement('video'));
+
+    expect(() => offscreen.clear()).not.toThrow();
+  });
+});