feat(macos): support VirtualHIDDevice v8

[malpern]

Summary

• update karabiner-driverkit from 0.3.x to 0.4.0
• update the lockfile to the published 0.4.0 crate
• document VirtualHIDDevice v8.0.0 as the supported macOS driver
• document the protocol-7 compatibility cut from protocol-5/v6.2.0

Why

Karabiner-Elements 16.1 uses VirtualHIDDevice v8. Kanata currently depends on the protocol-5 driverkit client and documents VirtualHIDDevice v6.2.0, so it cannot communicate with the current v8 daemon.

karabiner-driverkit 0.4.0 adds the protocol-7/v8 client support. This intentionally updates Kanata to v8 rather than maintaining two IPC implementations. Users of the standalone v6.2.0 driver must upgrade the driver and Kanata together.

Closes #2105.

Runtime validation

The driverkit migration was validated in a disposable macOS 15.7.7 ARM VM during psych3r/driverkit#20. The validated scenarios included:

• startup before and after daemon availability
• keyboard and pointing output readiness
• daemon loss and in-process recovery
• repeated reconnects
• clean shutdown
• two simultaneous output clients and recovery after daemon restart

The controlled v6/v8 comparison found the same same-device exclusive-input behavior in both versions; it was not introduced by the v8 migration.

Local checks

• cargo fmt --all --check
• RUSTFLAGS="-Dwarnings" cargo build --all-targets
• RUSTFLAGS="-Dwarnings" cargo test --all
• RUSTFLAGS="-Dwarnings" cargo clippy --all -- -D warnings
• RUSTFLAGS="-Dwarnings" cargo test --all --features=cmd
• RUSTFLAGS="-Dwarnings" cargo clippy --all --features=cmd -- -D warnings
• RUSTFLAGS="-Dwarnings" cargo build --release
• RUSTFLAGS="-Dwarnings" cargo build --release --features=cmd