feat(player): slideshow controller + <hyperframes-slideshow> component (#1581)

DOM-free SlideshowController (discrete nav, fragment holds, branch stack)
driving the existing player; <hyperframes-slideshow> web component with a
unified mute+nav capsule (conditional prev/next), floating hotspot overlays,
presenter mode (BroadcastChannel), keyboard/touch, and a scenes getter fed
via the runtime message handler.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/player/src/slideshow/test-setup.ts

@@ -0,0 +1,46 @@
+/**
+ * Vitest setup: install a minimal in-memory BroadcastChannel polyfill so that
+ * happy-dom tests can exercise the presenter/audience channel code path.
+ * This polyfill is intentionally NOT shipped in production code.
+ */
+
+type MsgHandler = (event: MessageEvent) => void;
+
+const registry = new Map<string, Set<InMemoryBroadcastChannel>>();
+
+class InMemoryBroadcastChannel {
+  onmessage: MsgHandler | null = null;
+  readonly name: string;
+  private _closed = false;
+
+  constructor(name: string) {
+    this.name = name;
+    let set = registry.get(name);
+    if (!set) {
+      set = new Set();
+      registry.set(name, set);
+    }
+    set.add(this);
+  }
+
+  // fallow-ignore-next-line complexity
+  postMessage(data: unknown): void {
+    if (this._closed) return;
+    const peers = registry.get(this.name);
+    if (!peers) return;
+    for (const peer of peers) {
+      if (peer === this) continue;
+      peer.onmessage?.(new MessageEvent("message", { data }));
+    }
+  }
+
+  close(): void {
+    if (this._closed) return;
+    this._closed = true;
+    registry.get(this.name)?.delete(this);
+  }
+}
+
+if (typeof globalThis.BroadcastChannel === "undefined") {
+  (globalThis as Record<string, unknown>)["BroadcastChannel"] = InMemoryBroadcastChannel;
+}