fix(selfhost): collection-based for-in with Std::Array + monomorphization fixes

- Fix Fmt_Format name conflict between lib/Fmt.bux and src/fmt.bux
  by renaming selfhost formatter function to Fmt_FormatSource.

- Fix collection-based for-in loops to handle already-monomorphized
  types like Array_int and Iter_string (typeArgCount=0 cases).

- Fix ekSizeOf and ekCast in HIR lowering to apply Lcx_SubstituteType
  during generic monomorphization, so sizeof(T) and *T casts emit
  correct concrete types in C output.

- Fix sema to infer stmt.refStmtType from initializer expression
  when no explicit type annotation is given, ensuring C backend
  emits correct pointer types for let data = ... as *T.

src/sema.bux

@@ -756,6 +756,15 @@ func Sema_CheckStmt(sema: *Sema, stmt: *Stmt) {
             } else {
                 sym.typeName = stmt.refStmtType.typeName;
             }
+        } else if stmt.child1 != null as *Expr && stmt.child1.refType != null as *TypeExpr {
+            // Infer type from initializer expression
+            sym.refType = stmt.child1.refType;
+            stmt.refStmtType = stmt.child1.refType;
+            if stmt.child1.refType.kind == tekPointer && stmt.child1.refType.pointerPointee != null as *TypeExpr {
+                sym.typeName = String_Concat(stmt.child1.refType.pointerPointee.typeName, "*");
+            } else {
+                sym.typeName = stmt.child1.refType.typeName;
+            }
         }
         sym.isMutable = stmt.boolValue;
         sym.isPublic = false;