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>

.gitignore

@@ -11,6 +11,8 @@ dist/
 clients/apple/.build/
 clients/apple/PunktfunkCore.xcframework/
 clients/apple/.swiftpm/
+# Generated App Store screenshots (tools/screenshots.sh output; uploaded as a CI artifact)
+clients/apple/screenshots/
 # Xcode per-user state
 xcuserdata/