Add UIKit live preview support for iOS 17

[RuslanTsitser]

Summary

• add iOS 17 support to the MarkdownEngine package with an editable UIKit/TextKit 2 NativeTextViewWrapper
• reuse the existing AST/tokenizer pipeline so iOS styling preserves the original Markdown source and UTF-16 ranges
• provide live styling for headings, emphasis, links, blockquotes, code, lists, task items, thematic breaks, tables, images, LaTeX source, and strikethrough
• reveal Markdown markers around the active construct while hiding them elsewhere
• preserve the existing AppKit implementation and behavior behind macOS platform guards
• isolate the existing macOS test suite and add focused UIKit styling tests

Implementation notes

The iOS editor uses the same public entry point as macOS and keeps editing and preview in one surface. It supports fit-to-content layout, selection-preserving restyling, list continuation, ordered-list increments, task reset on Return, raw-source mode, and accessibility identification.

The core target still has no new external dependencies. Existing AppKit-only service integrations and specialized renderers remain unchanged and macOS-only; the UIKit path styles their Markdown source without changing stored content.

Testing

• swift test — 470 tests in 68 suites passed on macOS
• xcodebuild iOS test action — MarkdownEngineIOSTests passed on iPhone 17 Pro Max Simulator
• MarkdownEngine builds for a generic iOS Simulator destination
• verified live editing in a real SwiftUI host: headings, bold, italic, hidden markers, and completed task styling