iOS: Ensure engine is run on the main thread (#191626)

Previously, running a `FlutterEngine` from a background queue silently
gave us an engine whose platform thread was not the main thread. The
first symptom an app sees is an assertion deep inside
`DisplayLinkManager.shared`, which is accurate but confusing and looks
like a bug rather than an intentional precondition.

`MakeThreadHost` adopts the calling thread as the engine's platform/UI
thread. That thread is required to be the main thread for several
reasons:
* it reads UIKit state and runs every plugin and platform channel
callback
* nothing else pumps its run loop

We now assert this via `FML_CHECK` in:
* `createShell:libraryURI:initialRoute:`: covers the `run*` methods and
`FlutterViewController`'s implicit engine
* `spawnWithEntrypoint:libraryURI:initialRoute:entrypointArgs:`: covers
the second and later engines in a `FlutterEngineGroup`.

A spawned engine inherits its spawner's task runners rather than
adopting the calling thread, so the check in `createShell` constrains
every engine in a group.

This also adds an `FML_DCHECK` during engine construction since it reads
`UIApplication` lifecycle state and registers UIKit observers.

This also annotates the Swift-visible entry points `NS_SWIFT_UI_ACTOR`
so that Swift callers get a compile-time error rather than a runtime
abort. `FlutterViewController` doesn't need an annotation since, like
all view controllers, it already inherits main actor isolation from
`UIViewController`.

Issue: https://github.com/flutter/flutter/issues/190909
Related: https://github.com/flutter/flutter/issues/190725
Related: https://github.com/flutter/flutter/issues/165904

No test changes, since these are assertions and unlike GoogleTest,
XCUITest and Swift Testing tests have no death-test. This doesn't add
any additional runtime aborts since `DisplayLinkManager.shared` already
does this; it just pushes them up to where they're actually actionable
by the user, and provides better error messages.

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