Rename to SwiftUICore/ComposeUI and extract reusable Kotlin libraries

[colemancda]

A structural refactor with no behavior change: clearer module boundaries on both the Swift and Kotlin sides, so the reusable pieces stand on their own and the demo is just a consumer. Verified end-to-end on the emulator and the desktop rig.

Swift modules

• AndroidSwiftUICore → SwiftUICore — the platform-neutral evaluation core (View/State/evaluator/RenderNode IR). No Android imports; builds and tests on any host.
• AndroidSwiftUIBridge → ComposeUI — the JNI materializer/bridge to the Compose interpreter. Platform-neutral (desktop JVM + Android).
• AndroidSwiftUI now @_exported imports both SwiftUICore and ComposeUI, and adds only the Android host (android.view bridging: MainActivity, Application, host view). App code keeps importing the one umbrella.

The demo's shared sources gate the desktop branch on canImport(ComposeUI) (a unique name) rather than canImport(SwiftUICore) — Apple ships a private SwiftUICore framework that would otherwise collide.

Kotlin libraries (extracted to repo root)

The Gradle build moved from Demo/ to the repo root, and the reusable Kotlin now lives in two root modules, with the demo consuming them:

• :composeui — the Compose Multiplatform interpreter (was Demo/swiftui, com.pureswift.swiftui): Render, ViewNode, TreeStore, the host view, the callback sink. Reusable CMP library (android + desktop).
• :androidbridge — a reusable Kotlin-only Android library (no Swift, no Compose) holding the JNI host glue: SwiftObject, NativeLibrary (now with a configurable library name), Runnable. Its classes stay in com.pureswift.swiftandroid for JNI symbol compatibility while the module namespace is com.pureswift.androidbridge.
• :demo-app (Demo/app) depends on both; :demo-desktop (Demo/desktop) depends on :composeui.

Verification

• swift test — 117 core tests pass under the renamed SwiftUICore.
• Both :composeui and :androidbridge build as AARs; the demo app builds against them and runs on the emulator (catalog renders; logcat shows the extracted NativeLibrary loading the Swift .so, proving the full JNI chain through the split-out glue).
• The macOS desktop rig builds and runs from the new root through SwiftUICore + ComposeUI (:demo-desktop:run), with the shared catalog rendering and a Button screen's tap counter incrementing — the full state loop.

Note

Task names changed: :app/:swiftui/:desktop → :demo-app/:composeui + :androidbridge/:demo-desktop, and gradle now runs from the repo root. This PR also carries the earlier desktop code-sharing work (the rig reuses the Android demo's playground sources, Android-only screens gated out).