Update to Swift 6.2 and Apple Platforms v26 (2025)

[Malith-Rukshan]

Summary

This PR updates swift-markdown-ui to support Swift 6.2 and the latest Apple platforms (iOS/macOS/tvOS/watchOS/visionOS v26) with full Swift concurrency compliance.

Motivation

With Apple's 2025 platform releases using v26 version numbering and Swift 6 introducing strict concurrency checking, this update ensures the library is compatible with
modern Swift development practices and the latest OS versions.

Changes

Platform Updates

• Swift tools version: 5.6 → 6.2
• iOS: 15.0 → 26.0
• macOS: 12.0 → 26.0
• tvOS: 15.0 → 26.0
• watchOS: 8.0 → 26.0
• macCatalyst: 15.0 → 26.0
• Added visionOS 26.0 support

Swift Concurrency

• Added Sendable conformance to all public types
• Applied proper @mainactor isolation for View-related code
• Fixed all Swift 6 strict concurrency warnings

Dependencies

• Updated NetworkImage, swift-snapshot-testing, and swift-cmark to latest versions

Demo App

• Updated deployment target to iOS 26.0

Testing

• ✅ Package builds with zero errors
• ✅ All concurrency checks pass
• ✅ Demo app verified on iOS 26 simulator
• ✅ Markdown rendering features working correctly

Breaking Changes

This PR increases minimum platform requirements to v26. Projects targeting earlier iOS/macOS versions should use the previous release.

Additional Notes

I have read and agree to follow the project's Code of Conduct.