scaffold: src/main/{infra,domain} directory tree for v5 split

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main/index.ts

@@ -0,0 +1,3 @@
+// Stammverzeichnis fuer das v5-Architektur-Refactoring.
+// Plan 04 macht daraus den Entry-Point statt src/main.ts.
+export {};