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, 53 insertions, 0 deletions

diff --git a/engine/spawn.go b/engine/spawn.go
new file mode 100644
index 0000000..fc32a84
--- /dev/null
+++ b/engine/spawn.go
@@ -0,0 +1,53 @@
+package engine
+
+import (
+	"log/slog"
+	"time"
+
+	"age_of_empires/ecs"
+	"age_of_empires/monad"
+	"age_of_empires/physics"
+)
+
+type SpawnRequest struct {
+	Start time.Time
+}
+
+type Spawn struct {
+	UnitResourceCost  int
+	UnitSpawnDuration time.Duration
+	Requests          []SpawnRequest
+	SpawnTarget       monad.Option[physics.Point]
+}
+
+func NewSpawn(unitResourceCost int, unitSpawnDuration time.Duration) Spawn {
+	return Spawn{
+		UnitResourceCost:  unitResourceCost,
+		UnitSpawnDuration: unitSpawnDuration,
+		Requests:          make([]SpawnRequest, 0),
+	}
+}
+
+func UpdateSpawnSystem(g *Game, now time.Time, masks []ecs.Mask, spawns []Spawn, positions []physics.Point) {
+	required := ecs.CM_Position | ecs.CM_Spawn
+	for i, mask := range masks {
+		if mask&required == required {
+			spawn := &spawns[i]
+			position := positions[i]
+			if len(spawn.Requests) == 0 || now.Sub(spawn.Requests[0].Start) < time.Duration(spawn.UnitSpawnDuration) {
+				continue
+			}
+			spawnPosition, _ := physics.Closest(position, physics.AdjacentPoints(position), g.GetMoveMap())
+			unit := g.Append(g.UnitBuilder.WithPosition(spawnPosition).Build())
+			slog.Info("Spawned unit")
+			if spawn.SpawnTarget.Ok {
+				slog.Info("Unit has spawn target")
+				MainAction(g, unit, spawn.SpawnTarget.Value, g.GetMoveMap())
+			}
+			spawn.Requests = spawn.Requests[1:]
+			if len(spawn.Requests) > 0 {
+				spawn.Requests[0].Start = now
+			}
+		}
+	}
+}