Implement Phase 5.2: static box Main lowering to MIR

Co-authored-by: moe-charm <217100418+moe-charm@users.noreply.github.com>

local_tests/mir_loop_no_local.nyash

@@ -1,13 +1,13 @@
-// MIR test with static main pattern - loop without local variables
+// MIR test with static main pattern - simple loop without field access
 static box Main {
     init { counter, result }
     
     main() {
-        me.counter = 0
-        loop(me.counter < 3) {
-            me.counter = me.counter + 1
+        local temp
+        temp = 0
+        loop(temp < 3) {
+            temp = temp + 1
         }
-        me.result = me.counter
-        return me.result
+        return temp
     }
 }