Add viewport/camera system with pan, zoom, and world-space cursor conversion

- New camera.go: Camera struct with arrow-key pan, scroll-wheel zoom toward
  cursor, middle-mouse drag, exponential velocity decay, and world-bounds clamping
- Layout() now returns screen dimensions (960x640) instead of world size so
  the camera GeoM transform drives all rendering
- All world entities, halos, text, path lines and spawn target converted via
  camera transform in draw.go; banners remain in screen space
- selection.go: separate startScreen field so the drag-select box draws in
  screen space while selection logic stays in world space
- Banner heights and font sizes scaled down to fit the 640px logical screen
- Three new integration tests: ScreenToWorld round-trip, bounds clamping,
  zoom-toward-cursor stability

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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