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/runtime-message-handler.test.ts

@@ -18,6 +18,7 @@ const makeCallbacks = (): MessageHandlerCallbacks => ({
   media: { mirrorTime: vi.fn(), promoteToParentProxy: vi.fn() } as unknown as ParentMediaManager,
   getPlaybackState: vi.fn(() => ({ currentTime: 0, duration: 0, paused: true, lastUpdateMs: 0 })),
   setPlaybackState: vi.fn(),
+  setScenes: vi.fn(),
   getShaderLoadingMode: vi.fn(() => "auto"),
   shaderLoader: { update: vi.fn() } as unknown as ShaderLoaderState,
   setCompositionSize: vi.fn(),