Bump Package.swift to Swift 6.2 and update the recommended Xcode version to 26.3

[Kyle-Ye]

Summary

Bump Package.swift to Swift tools 6.2 and update the recommended development environment to Xcode 26.3.

Motivation

Package.swift currently uses Swift tools 6.1, and the README/recommended setup should point contributors at Xcode 26.3 / Swift 6.2. Swift 6.2 also includes package, build, and testing improvements that are worth adopting where they fit OpenSwiftUI.

Scope

• Update Package.swift to Swift tools 6.2.
• Update the recommended Xcode version to Xcode 26.3.
• Verify package resolution and builds with Xcode 26.3 / Swift 6.2.
• Review Swift 6.2 manifest APIs such as SwiftSetting.treatWarning / treatAllWarnings.
• Check whether macro targets benefit from SwiftPM's prebuilt swift-syntax support.
• Adopt Swift Testing 6.2 improvements where useful:
  • Add exit tests for intentional preconditionFailure, fatalError, or exit paths.
  • Revisit skipped/TODO crash tests such as the LayoutTraitsTests exit-test placeholder.
  • Consider attachments for UI/snapshot/rendering test diagnostics.
  • Use raw identifier display names where they improve test readability.
• Keep the Swift language mode decision explicit; do not silently migrate all targets to Swift 6 language mode.

Validation

• swift package resolve
• swift build
• Relevant SwiftPM test targets
• Existing macOS/iOS CI workflows where available
• Confirm older supported build paths are not unintentionally broken