feat(apple): App Store screenshot harness + CI zip artifact

A DEBUG-only "shot mode" renders one mock-populated screen full-bleed
(PUNKTFUNK_SHOT_SCENE=<name> -> ScreenshotHostView instead of ContentView),
so the OS can screenshot the REAL, fully-rendered UI. tools/screenshots.sh
drives it: screencapture for the mac window, `simctl io booted screenshot`
for the iOS/iPad/tvOS Simulators, at exactly the App Store Connect sizes.

ImageRenderer was tried first and rejected: it can't rasterize this app's
chrome (NavigationStack, Form/TabView, Liquid-Glass/NSVisualEffect all render
black or the "can't render" placeholder). Capturing the live window/Simulator
avoids that. Only the stream hero is synthetic (StreamView needs a live
connection) - a synthwave frame + the real glass HUD, overridable via
PUNKTFUNK_SHOT_HERO.

CI: a new `screenshots` job in apple.yml builds the iOS (+ tvOS best-effort)
xcframework slices, runs the harness per platform best-effort, and attaches
the result as a single zip artifact (punktfunk-appstore-screenshots). It is
isolated from the build/test job and skipped on PRs, so a capture gap (missing
Simulator runtime, or no Screen Recording grant for the mac window capture)
never reds the core signal.

Generated PNGs (clients/apple/screenshots/) are gitignored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/PunktfunkClientApp.swift

@@ -14,7 +14,18 @@ struct PunktfunkClientApp: App {
 
     var body: some Scene {
         WindowGroup("Punktfunk") {
+            #if DEBUG
+            // PUNKTFUNK_SHOT_SCENE=<name> → show that single mock-populated screen full-bleed for
+            // the App Store screenshot capture (tools/screenshots.sh). Normal launch otherwise;
+            // the whole path is absent from Release builds.
+            if let scene = ScreenshotMode.requestedScene {
+                ScreenshotHostView(scene: scene)
+            } else {
+                ContentView()
+            }
+            #else
             ContentView()
+            #endif
         }
         // The Stream menu (Disconnect ⌘D, Show/Hide Statistics ⌘⇧S) — a real menu bar on
         // macOS, hardware-keyboard shortcuts on iPad. tvOS has neither.