feat(galaxy): refactor params into core/disk/beam layers, fix randomize button scaling

- Split GalaxyParams into CoreParams, DiskParams, and BeamParams sub-structs
- Add polar beam systems (top/bottom) with visual differentiation
- Add scrollable control panel with section headers for each layer
- Fix Randomize Settings button: remove explicit width (use flex auto)
  and bump height from 28px to 32px to match other action buttons
- Add mouse-wheel scroll routing for the control panel
- Add 'Randomize Settings' button that randomizes all params within bounds
- Add SystemOrigin enum for per-layer tracking
- Update AGENTS.md with new module layout and plugin pattern

apps/game/src/camera.rs

@@ -111,6 +111,9 @@ pub fn orbit_camera_control(
     }
 
     for event in scroll_events.read() {
+        if cursor_over_ui {
+            continue;
+        }
         // Scroll up (positive y) → decrease distance (zoom in).
         orbit.distance *= 1.0 - event.y * ORBIT_ZOOM_SENSITIVITY;
     }