chore: support macOS 15 and Swift 6.2

[lynnswap]

Summary

• Lower the Swift package manifest from Swift tools 6.3 to 6.2 and the package platform requirement from macOS 26 to macOS 15.
• Update README requirements to document macOS 15.0+ and Swift 6.2+ support.
• Lower the MiniApp macOS deployment target to 15.0 for manual verification.
• Add GitHub Actions CI for Swift 6.2 on macOS 15 and latest Xcode on macOS 26.

Testing

• swift test
• git diff --check
• CI YAML parse check
• Xcode Issue Navigator warning check
• codex-review