feat: let mut + 赋值语句 — while 循环可修改变量

- lexer: 新增 TOK_MUT 关键字
- ast: AST_ASSIGN_STMT 节点 + let_stmt.is_mut 标志
- parser: ‘let mut’ 前缀识别 + ‘ident = expr;’ 赋值语句
- sema: Symbol.is_mut 可变性检查（不可变变量赋值报错）
- codegen: AST_ASSIGN_STMT → store 指令
- 新增集成测试 06_mut_while.l（while 循环 + 计数器）

基于 Codex 分析报告 P0 建议。

src/sema/sema.c

@@ -204,13 +204,48 @@ static void analyze_node(AstNode* node, Scope* scope, ErrorList* errors, Arena*
         }
 
         node->type.kind = var_type;
-        if (!scope_insert(scope, a, node->as.let_stmt.name, SYM_VARIABLE, var_type)) {
+        Symbol* sym = scope_insert(scope, a, node->as.let_stmt.name, SYM_VARIABLE, var_type);
+        if (!sym) {
             error_add(errors, "<sema>", node->line, node->col,
                       "变量 '%s' 重复定义", node->as.let_stmt.name);
+        } else {
+            sym->is_mut = node->as.let_stmt.is_mut;
         }
         break;
     }
 
+    case AST_ASSIGN_STMT: {
+        Symbol* sym = scope_lookup(scope, node->as.assign_stmt.name);
+        if (!sym) {
+            error_add(errors, "<sema>", node->line, node->col,
+                      "未定义的变量 '%s'", node->as.assign_stmt.name);
+            node->type.kind = TYPE_ERROR;
+            break;
+        }
+        if (sym->kind != SYM_VARIABLE) {
+            error_add(errors, "<sema>", node->line, node->col,
+                      "'%s' 不是变量，不能赋值", node->as.assign_stmt.name);
+            node->type.kind = TYPE_ERROR;
+            break;
+        }
+        if (!sym->is_mut) {
+            error_add(errors, "<sema>", node->line, node->col,
+                      "不能对不可变变量 '%s' 赋值（需用 let mut 声明）",
+                      node->as.assign_stmt.name);
+            node->type.kind = TYPE_ERROR;
+            break;
+        }
+        analyze_expr(node->as.assign_stmt.value, scope, errors, a);
+        TypeKind value_ty = node->as.assign_stmt.value->type.kind;
+        if (value_ty != TYPE_ERROR && value_ty != sym->type) {
+            error_add(errors, "<sema>", node->line, node->col,
+                      "赋值类型不匹配: 变量 '%s' 类型为 '%s'，但表达式类型为 '%s'",
+                      node->as.assign_stmt.name, type_name(sym->type), type_name(value_ty));
+        }
+        node->type.kind = TYPE_VOID;
+        break;
+    }
+
     case AST_IF_STMT:
         analyze_expr(node->as.if_stmt.cond, scope, errors, a);
         if (node->as.if_stmt.cond->type.kind != TYPE_BOOL &&