engine: Config, scenario runner, golden regression tests; demo CLI

Config is the single source of truth for parameters (TS types will be
generated from these structs in milestone 2); all randomness flows from
its three seeds, so identical configs give identical results. Golden
test pins the default world: none=99/120 (82%), random=70/120 (58%),
most-connected=7/120 (6%). Same story as the prototype's 85/64/8 with
different dice; the ordering and the collapse are asserted explicitly,
exact Python numbers are out of scope by design.
'go run ./cmd/spreadlab' prints the three-scenario comparison.
This completes milestone 1 (engine ported, parameterised, tested).

cmd/spreadlab/main.go

@@ -1,9 +1,35 @@
-// Command spreadlab will serve the spreadlab dashboard. For now it only
-// proves the module builds; the HTTP server arrives in milestone 2.
+// Command spreadlab will serve the spreadlab dashboard. Until the HTTP
+// server lands (milestone 2), it runs the prototype's three scenarios in
+// the default world and prints the comparison.
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"os"
+
+	"github.com/JustinZeus/spreadlab/internal/engine"
+)
 
 func main() {
-	fmt.Println("spreadlab: engine under construction (milestone 1)")
+	config := engine.DefaultConfig()
+	fmt.Printf("spreadlab: %d students, educate %d, forwarding probability %.2f\n",
+		config.NumStudents, config.NumEducated, config.ForwardProb)
+	fmt.Println("(illustrative, not validated)")
+	fmt.Println()
+
+	strategies := []engine.Strategy{
+		engine.StrategyNone,
+		engine.StrategyRandom,
+		engine.StrategyMostConnected,
+	}
+	for _, strategy := range strategies {
+		result, err := engine.RunScenario(config, strategy)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "spreadlab:", err)
+			os.Exit(1)
+		}
+		fmt.Printf("%-15s educated=%3d reached=%3d/%d (%2.0f%%) in %d rounds\n",
+			result.Strategy, len(result.Educated), result.NumReached,
+			config.NumStudents, result.ReachedPct, result.NumRounds)
+	}
 }