Move the App setup and module declarations from src/main.rs into a new
src/lib.rs that exposes a single pub fn run(). src/main.rs becomes a
three-line entry point that calls bglga::run(). This is the standard
Bevy-book pattern and lets the game logic be consumed as a library
(e.g. for integration tests, headless tooling, or alternate entry
points) without duplicating the App wiring.
Also gate update_tractor_beam_visual with
run_if(any_with_component::<TractorBeam>) so the system only runs while
a boss has an active tractor beam, instead of every Update tick.
