Rust rewrite

Changes

• Replaces the Swift executable with Rust source under src/
• Adds Cargo.toml, Cargo.lock, and Cargo macOS build config
• Reimplements Caps Lock LED control through macOS CoreFoundation/CoreGraphics/IOKit FFI
• Reimplements network interface byte polling and existing CLI flag behavior
• Adds CLI parser unit coverage
• Removes SwiftPM and Xcode project files
• Updates README/changelog references from Swift to Rust

Validation

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test --locked
• cargo build --release --locked
• target/release/netcaps --version
• target/release/netcaps --help

Note .github/workflows/release.yml should be updated to build/package with cargo build --release --locked instead of swift build.