chore(deps): upgrade Bevy 0.13 → 0.18

Three major versions of breaking changes:
- Bundles → Required Components (SpriteBundle, NodeBundle, ButtonBundle,
  TextBundle, Camera2dBundle removed; spawn tuples of components instead)
- Style merged into Node; TextStyle split into TextFont + TextColor
- Color API: rgb/rgba → srgb/srgba; Color::Rgba pattern matching replaced
  with .alpha()/.set_alpha(); .r()/.g()/.b() → .to_srgba().red/green/blue
- Time: delta_seconds() → delta_secs(); elapsed_seconds() → elapsed_secs()
- Query: get_single()/get_single_mut() → single()/single_mut() (now Result)
- Timer::finished() → Timer::is_finished()
- despawn_recursive() removed (despawn() is now recursive); despawn_descendants()
  removed — replaced with Children query iteration
- BorderColor(c) → BorderColor::all(c)
- WindowResolution: From<(f32,f32)> removed → cast to (u32,u32)
- flake.nix: added wayland to runtime libs (default-on in 0.18)

Tests pass (8/8), clippy clean, headless render verified showing start
menu, button, starfield, and player ship.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flake.nix

@@ -26,6 +26,7 @@
           libxi
           libxkbcommon
           libxcb
+          wayland
           vulkan-loader
           glfw
         ];