Test and document the MemberMedia track list

Unit tests for tracks$ and trackBySource$, an end-to-end check of isActive$ through a mute cycle (skipped on Chromium, whose fake microphone the SFU never hears), the architecture document brought in line with the track list, and the changed behaviour recorded in the plan.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

sdk/src/utils/tracks.test.ts

@@ -0,0 +1,41 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { BehaviorSubject } from "rxjs";
+import { describe, expect, it } from "vitest";
+
+import { type MediaTrack } from "../api";
+import { testScope } from "./test";
+import { trackBySource$ } from "./tracks";
+
+const camera = { source: "camera", kind: "video", id: "1" } as MediaTrack;
+const microphone = {
+  source: "microphone",
+  kind: "audio",
+  id: "2",
+} as MediaTrack;
+
+describe("trackBySource$", () => {
+  it("picks the track of a source and follows the list", () => {
+    const tracks$ = new BehaviorSubject<MediaTrack[]>([]);
+    const camera$ = trackBySource$(testScope(), tracks$, "camera");
+    expect(camera$.value).toBeUndefined();
+    tracks$.next([microphone, camera]);
+    expect(camera$.value).toBe(camera);
+    tracks$.next([microphone]);
+    expect(camera$.value).toBeUndefined();
+  });
+
+  it("does not emit when the list changes elsewhere", () => {
+    const tracks$ = new BehaviorSubject<MediaTrack[]>([camera]);
+    const camera$ = trackBySource$(testScope(), tracks$, "camera");
+    const emissions: unknown[] = [];
+    camera$.subscribe((t) => emissions.push(t));
+    tracks$.next([camera, microphone]);
+    expect(emissions).toEqual([camera]);
+  });
+});