stage3: unify to cleanup; MIR return-defer; docs+smokes updated; LLVM(harness): finalize_phis ownership, ret.py simplified, uses-predeclare; cleanup return override green; method-postfix cleanup return WIP (PHI head)

apps/tests/try_finally_normal.nyash

@@ -4,10 +4,9 @@ static box Main {
     try {
       console.println("T")
       return 123
-    } finally {
+    } cleanup {
       // finally は必ず実行される想定
       console.println("F")
     }
   }
 }
-