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>

1 files changed, 11 insertions, 6 deletions

diff --git a/selection/selection.go b/selection/selection.go
index e4afdaa..1c702ee 100644
--- a/selection/selection.go
+++ b/selection/selection.go
@@ -32,10 +32,11 @@ type selectItem struct {
 }
 
 type GlobalSelection struct {
-	isActive bool
-	start    physics.Point
-	buffer   []selectItem
-	Selected []ecs.Entity
+	isActive    bool
+	start       physics.Point
+	startScreen physics.Point
+	buffer      []selectItem
+	Selected    []ecs.Entity
 }
 
 func (s GlobalSelection) IsActive(cursor physics.Point) bool {
@@ -56,6 +57,10 @@ func (s *GlobalSelection) Start(point physics.Point) {
 	s.start = point
 }
 
+func (s *GlobalSelection) StartScreen(point physics.Point) {
+	s.startScreen = point
+}
+
 type SelectTarget struct {
 	Selectable *Selectable
 	Position   physics.Point
@@ -134,6 +139,6 @@ func (s *GlobalSelection) Unselect(selectables []Selectable) {
 	s.Selected = []ecs.Entity{}
 }
 
-func (s *GlobalSelection) Draw(dst *ebiten.Image, cursor physics.Point) {
-	vector.StrokeRect(dst, float32(s.start.X), float32(s.start.Y), float32(cursor.X-s.start.X), float32(cursor.Y-s.start.Y), 10.0, color.RGBA{256 * 3 / 16, 256 * 3 / 16, 256 * 3 / 16, 256 / 4}, true)
+func (s *GlobalSelection) Draw(dst *ebiten.Image, screenCursor physics.Point) {
+	vector.StrokeRect(dst, float32(s.startScreen.X), float32(s.startScreen.Y), float32(screenCursor.X-s.startScreen.X), float32(screenCursor.Y-s.startScreen.Y), 10.0, color.RGBA{256 * 3 / 16, 256 * 3 / 16, 256 * 3 / 16, 256 / 4}, true)
 }