Adds Linux support

[mmllr]

This PR adds support for building on Linux with a few minor changes by addressing platform-specific incompatibilities:

• Wrap TIOCGWINSZ and termios flag constants (e.g. ECHO, ICANON) in .init() to resolve Linux C-interop type mismatches (on Linux it is UInt32 vs UInt on macOS).
• Add @preconcurrency to Foundation in Cursor.swift to avoid strict concurrency warnings on Linux
• Skip XCTest performance benchmarks (os(Darwin)), copyToClipboard(), and remove the AppKit import — all macOS-specific