Tractor example

Use the tractor examples when you need a tractor-side role with GNSS, diagnostics, implement command, or facility behavior.

Look for:

• examples/tractor_ecu_demo.rs
• examples/session_minimal.rs plus presets::tractor() (see The session facade)

Expected learning:

• how a tractor node claims an address
• how tractor-side subsystems are plugged in (or pulled in with presets::tractor())
• how events are drained