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

1 files changed, 42 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())
+	}
+}