MOB-97: port UIScene lifecycle adoption (Xcode 27 requirement)

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Summary

Xcode 27 requires scene-based app startup; the current template boots everything in AppDelegate.application:didFinishLaunchingWithOptions:, which no longer works. Ports the fix already verified externally in the downstream app (commits 13dc2ab + 75ff1aa):

• AppDelegate: drops the window property, didFinishLaunchingWithOptions: reduced to a bare return YES;. Every other method (orientation mask, push-token handlers) untouched.
• New SceneDelegate (same file): scene:willConnectToSession:options: creates the window via initWithWindowScene: and sets rootViewController/makeKeyAndVisible on every call (a scene can disconnect/reconnect without relaunching the process) — but wraps mob_register_plugins()/mob_init_ui()/the BEAM-boot pthread in a static dispatch_once_t, since a second erl_start in the same process is fatal and nothing in mob's native layer guards against it.
• Info.plist: UISceneConfigurations added alongside the existing UIApplicationSupportsMultipleScenes=false.

Test plan

• mix test — 319 passed (the existing AppDelegate.m content assertions are substring checks, order-independent of which class contains the code — no test changes needed)
• mix test --only lint
• Built a dev archive from this branch, generated a fresh app, and ran clang -fsyntax-only against the real rendered AppDelegate.m on iOS Simulator SDK 26.5 (Xcode 27 itself isn't available on this machine) — zero warnings/errors, and confirms the old initWithFrame:/UIScreen.mainScreen deprecation warnings are gone
• Actual Xcode 27 build — can't verify, not installed here

Linear: MOB-97

🤖 Generated with Claude Code

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Addressed both gaps from review:

1. Generator test pinning the invariants — project_generator_test.exs now asserts the rendered AppDelegate.m declares UIWindowSceneDelegate and calls dispatch_once(&<token>, ...) at the actual call site (deliberately not just the bare "dispatch_once" substring, which also matches the unrelated dispatch_once_t type declaration and would miss someone removing just the call). Verified this actually catches the regression: temporarily stripped the dispatch_once(...) call from the template, confirmed the test fails with the expected message, restored it, confirmed green again. Also asserts Info.plist's UISceneDelegateClassName.

2. Existing-apps caveat — decisions/2026-08-25-uiscene-lifecycle-xcode27.md, following the same pattern as 2026-06-17-android-16kb-page-size.md: explicitly states AppDelegate.m/Info.plist are app-owned and copied at generation, so existing apps need a hand-port (not picked up by a dep bump), and points at MOB-97 + the downstream app's commits for anyone doing that port.

Pushed as a new commit on this branch.