feat(apple/M0): App Group + PunktfunkShared + punktfunk:// deep links

Foundation milestone for Live Activities & Widgets (design/apple-live-
activities-and-widgets.md). No user-visible change beyond the URL scheme.

- New dependency-free PunktfunkShared SwiftPM target (+ library product) so a
  future widget extension can link it WITHOUT PunktfunkKit (Rust staticlib +
  presentation layer). Moves StoredHost (model + JSON codec), DefaultsKeys, and
  punktfunkDefaultMgmtPort there; adds AppGroup.suiteName and the punktfunk://
  DeepLink builder/parser. PunktfunkKit @_exported-imports it (no call-site
  churn for consumers; intra-Kit files import it explicitly since imports are
  file-scoped).
- HostStore reads/writes the shared App-Group suite (group.io.unom.punktfunk)
  with a one-time migration from UserDefaults.standard (old value left in place
  for staged rollout); reloads the "PunktfunkHosts" widget timeline on change.
- App Group entitlement on iOS/tvOS + macOS.
- CFBundleURLTypes scheme `punktfunk`; ContentView.onOpenURL routes
  connect/<uuid>[?launch=<GameEntry.id>] into the existing connect() path
  (unknown host / already-streaming guards; never tears down a live session).
- Round-trip tests: StoredHost JSON codec (+ legacy missing-optional decode),
  DeepLink grammar. `swift build` + `swift test` green (142 tests, 0 failures).

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

clients/apple/Config/Info.plist

@@ -19,5 +19,19 @@
 	<array>
 		<string>_punktfunk._udp</string>
 	</array>
+	<!-- Deep links: punktfunk://connect/<host-uuid>[?launch=<GameEntry.id>]. Emitted by the
+	     launcher widget and Siri/Shortcuts; routed by ContentView.onOpenURL into the existing
+	     connect path. Shared across all three targets (tvOS/macOS accept it harmlessly). -->
+	<key>CFBundleURLTypes</key>
+	<array>
+		<dict>
+			<key>CFBundleURLName</key>
+			<string>io.unom.punktfunk.deeplink</string>
+			<key>CFBundleURLSchemes</key>
+			<array>
+				<string>punktfunk</string>
+			</array>
+		</dict>
+	</array>
 </dict>
 </plist>