Add examples catalog with subcommand dispatch

- Extract game engine from package main to engine/ library package
- Create examples/ package with registry, Entry type, and BuilderDeps
- Register default world as an example (moved from main())
- Add combat example: two teams of units facing each other
- Rewrite main.go as thin dispatch: go run . example [name]
- Update README with examples catalog one-liner

3 files changed, 102 insertions, 0 deletions

diff --git a/examples/combat.go b/examples/combat.go
new file mode 100644
index 0000000..f2dd92f
--- /dev/null
+++ b/examples/combat.go
@@ -0,0 +1,42 @@
+package examples
+
+import (
+	"image/color"
+
+	"age_of_empires/engine"
+	"age_of_empires/physics"
+	"age_of_empires/selection"
+)
+
+func init() {
+	Registry["combat"] = Entry{
+		Name:        "Combat",
+		Description: "Two groups of units facing each other — ready for battle",
+		Build:       buildCombat,
+	}
+}
+
+func buildCombat(g *engine.Game, deps BuilderDeps) {
+	g.ResourceAmount = 500
+	g.Camera = engine.Camera{X: 800, Y: 1000, Zoom: 1.0}
+	g.CurrentAction = engine.Selecting
+	g.UnitBuilder = deps.UnitBuilder
+
+	// Red team (left)
+	redBuilder := engine.NewEntityBuilder().
+		WithSolid(engine.NewFilledCircleImage(100, color.RGBA{0xff, 0x00, 0x00, 0xff})).
+		WithSelectable("round", selection.Unit).
+		WithMove().WithOrder().WithResourceGatherer(15)
+	for x := 0; x < 5; x++ {
+		g.Append(redBuilder.WithPosition(physics.Point{X: 200 + x*120, Y: 1000}).Build())
+	}
+
+	// Blue team (right)
+	blueBuilder := engine.NewEntityBuilder().
+		WithSolid(engine.NewFilledCircleImage(100, color.RGBA{0x00, 0x00, 0xff, 0xff})).
+		WithSelectable("round", selection.Unit).
+		WithMove().WithOrder().WithResourceGatherer(15)
+	for x := 0; x < 5; x++ {
+		g.Append(blueBuilder.WithPosition(physics.Point{X: 1000 + x*120, Y: 1000}).Build())
+	}
+}
diff --git a/examples/default.go b/examples/default.go
new file mode 100644
index 0000000..d291a26
--- /dev/null
+++ b/examples/default.go
@@ -0,0 +1,40 @@
+package examples
+
+import (
+	"image/color"
+	"time"
+
+	"age_of_empires/engine"
+	"age_of_empires/physics"
+	"age_of_empires/selection"
+)
+
+func init() {
+	Registry["default"] = Entry{
+		Name:        "Default",
+		Description: "Free play with a town center, iron mine, and gatherer units",
+		Build:       buildDefault,
+	}
+}
+
+func buildDefault(g *engine.Game, deps BuilderDeps) {
+	g.ResourceAmount = 150
+	g.Camera = engine.Camera{X: 1000, Y: 1000, Zoom: 1.0}
+	g.CurrentAction = engine.Selecting
+	g.UnitBuilder = deps.UnitBuilder
+
+	ironMineBuilder := engine.NewEntityBuilder().
+		WithSolid(engine.NewFilledRectangleImage(physics.Point{X: 100, Y: 100}, color.RGBA{0x80, 0x80, 0x80, 0xff})).
+		WithResourceSource(1000).
+		WithSelectable("square", selection.Building)
+	g.Append(ironMineBuilder.WithPosition(physics.Point{X: 1000, Y: 1000}).Build())
+
+	townCenterBuilder := engine.NewEntityBuilder().
+		WithSolid(engine.NewFilledRectangleImage(physics.Point{X: 100, Y: 100}, color.RGBA{0x0, 0x0, 0xff, 0xff})).
+		WithResourceStorage().
+		WithSelectable("square", selection.Building).
+		WithSpawn(engine.NewSpawn(50, 1*time.Second))
+	townCenter := g.Append(townCenterBuilder.WithPosition(physics.Point{X: 1000, Y: 2000}).Build())
+
+	_ = townCenter // suppress unused warning; future spawn loop will use it
+}
diff --git a/examples/examples.go b/examples/examples.go
new file mode 100644
index 0000000..9e18d6b
--- /dev/null
+++ b/examples/examples.go
@@ -0,0 +1,20 @@
+package examples
+
+import (
+	"age_of_empires/engine"
+
+	"github.com/hajimehoshi/ebiten/v2/text/v2"
+)
+
+type BuilderDeps struct {
+	FaceSource  *text.GoTextFaceSource
+	UnitBuilder engine.EntityBuilder
+}
+
+type Entry struct {
+	Name        string
+	Description string
+	Build       func(*engine.Game, BuilderDeps)
+}
+
+var Registry = map[string]Entry{}