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, 107 insertions, 0 deletions

diff --git a/engine/entity.go b/engine/entity.go
new file mode 100644
index 0000000..7aa43b1
--- /dev/null
+++ b/engine/entity.go
@@ -0,0 +1,107 @@
+package engine
+
+import (
+	"image/color"
+
+	"age_of_empires/ecs"
+	"age_of_empires/physics"
+	"age_of_empires/selection"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+type Components struct {
+	Position         physics.Point
+	RelBounds        physics.Rectangle
+	Image            *ebiten.Image
+	Selectable        selection.Selectable
+	Move             physics.Move
+	Order            OrderKind
+	ResourceGatherer ResourceGatherer
+	ResourceSource   ResourceSource
+	ResourceStorage  ResourceStorage
+	Spawn            Spawn
+}
+
+type EntityBuilder struct {
+	mask       ecs.Mask
+	components Components
+}
+
+func NewEntityBuilder() EntityBuilder {
+	return EntityBuilder{
+		components: Components{},
+	}
+}
+
+func (b EntityBuilder) WithPosition(position physics.Point) EntityBuilder {
+	b.components.Position = position
+	b.mask |= ecs.CM_Position
+	return b
+}
+
+func (b EntityBuilder) WithSolid(image *ebiten.Image) EntityBuilder {
+	b.components.Image = image
+	b.mask |= ecs.CM_Image
+	b.components.RelBounds = image.Bounds()
+	b.mask |= ecs.CM_RelBounds
+	return b
+}
+
+var Red = color.RGBA{0xff, 0x00, 0x00, 0xff}
+
+func (b EntityBuilder) WithSelectable(haloKind string, priority selection.Priority) EntityBuilder {
+	b.components.Selectable = selection.Selectable{
+		IsSelected: false,
+		Halo:       nil,
+		Priority:   priority,
+	}
+	b.mask |= ecs.CM_Selectable
+	switch haloKind {
+	case "round":
+		b.components.Selectable.Halo = NewStrokeCircleImage(110, selection.HaloWidth, Red)
+	case "square":
+		b.components.Selectable.Halo = NewStrokeRectangleImage(physics.Point{X: 110, Y: 110}, selection.HaloWidth, Red)
+	}
+	return b
+}
+
+func (b EntityBuilder) WithResourceStorage() EntityBuilder {
+	b.components.ResourceStorage = ResourceStorage{}
+	b.mask |= ecs.CM_ResourceStorage
+	return b
+}
+
+func (b EntityBuilder) WithResourceSource(amount int) EntityBuilder {
+	b.components.ResourceSource = ResourceSource{Remaining: amount}
+	b.mask |= ecs.CM_ResourceSource
+	return b
+}
+
+func (b EntityBuilder) WithSpawn(spawn Spawn) EntityBuilder {
+	b.components.Spawn = spawn
+	b.mask |= ecs.CM_Spawn
+	return b
+}
+
+func (b EntityBuilder) WithMove() EntityBuilder {
+	b.components.Move = physics.Move{IsActive: false}
+	b.mask |= ecs.CM_Move
+	return b
+}
+
+func (b EntityBuilder) WithOrder() EntityBuilder {
+	b.components.Order = OrderKindNone
+	b.mask |= ecs.CM_Order
+	return b
+}
+
+func (b EntityBuilder) WithResourceGatherer(maxCapacity int) EntityBuilder {
+	b.components.ResourceGatherer = ResourceGatherer{MaxCapacity: maxCapacity}
+	b.mask |= ecs.CM_ResourceGatherer
+	return b
+}
+
+func (b EntityBuilder) Build() (Components, ecs.Mask) {
+	return b.components, b.mask
+}