feat: embed GPUI Kit Markdown in a Swift iOS container

[huacnlee]

Summary

Embed GPUI Kit's Markdown TextView in a Swift-owned UIKit window instead of letting GPUI create the iOS example's window. A child controller renders into GPUITextView: UIView; native Compact / Expanded controls exercise layout changes. The light English sample puts emphasis, quotes, nested lists, code, tables, and tasks on the first screen.

Fix two startup failures exposed by the simulator: supply the UIKit display handle to wgpu 29, and retain the run_embedded application handle so native callbacks cannot outlive the GPUI app. Synchronize layout and redraw without implicit animations during size changes. Correct the Xcode static-library paths and build directory.

Pins GPUI Kit to commit 7d9efcd2069f9eaa6eb3ba6345aac4aa7d87c9f7 from longbridge/gpui-kit#3045, so a sibling local checkout is not required. No crates are published.

The embedding example supports one GPUI view for the application lifetime; independent destruction and multiple instances are not implemented.

Test Plan

• Passed cargo check --manifest-path example/Cargo.toml --target aarch64-apple-ios-sim with the pinned Git dependency.
• Built the Swift app with Xcode for the iOS simulator and ran it on iPhone 17 Pro / iOS 26.5 using the same GPUI Kit source revision locally.
• Verified light Markdown rendering, native controls, and Compact / Expanded resizing.
• Passed git diff --check and bash -n example/build.sh.
• Embedded scrolling needs manual verification: the automation tool emitted touch begin/end without move events. Transient resize jitter also needs visual confirmation.
• No Android, physical-device, or performance testing performed.