Add linked demo, workspace, and simulator UI tests

[cysp]

Summary

Add a small iOS 15 demo app in Demo/STTouchDisplayDemo.xcodeproj. It links the existing STTouchDisplay static-library target through an explicit Xcode project reference. The root STTouchDisplay.xcodeproj remains the library and unit-test project. A root STTouchDisplay.xcworkspace opens both projects together.

The single commit adds the scene-based demo app, project reference, workspace, documentation, and demo build, UI-test, and analysis scripts used locally and in CI. The UI test checks the button's updated title and touch status to verify button interaction and marker creation/removal.

Related changes

Both #5 and this PR target master independently after #4 merged. This PR does not include #5's public radius change. The demo and its UI tests live together here. Swift Package Manager remains deferred.

Validation

• Local Xcode 27 on the master-based head: Objective-C formatting and the demo UI test passed.
• Project, scheme, Info.plist, and workspace syntax checks passed; the workspace exposes the library and demo schemes.
• Xcode 26.6 CI passed on commit 145217f, including formatting, Release builds, Debug unit and UI tests, and both static analyzers.

Review note

The UI test observes demo status derived from the overlay's marker views. It checks event delivery, button interaction, and marker creation/removal; it does not measure the marker's appearance or physical contact geometry.