feat: 闭包/lambda — 匿名函数表达式

fn(x: T) -> R { body } 作为表达式, 可赋值给变量并间接调用。

全流水线实现:
- Parser: TOK_FN 前缀 → AST_LAMBDA 节点
- Sema: 自动生成 __lambda_N 顶层函数 + 符号注册
- Sema: analyze_call_expr 支持 TYPE_CLOSURE 变量调用
- Codegen: lambda 表达式返回函数指针(i64), 调用点载入+IntToPtr+间接call
- VarEntry.closure_fn 追踪闭包变量对应的生成函数

限制(MVP v0.1): 非捕获 lambda, 返回类型固定 i64

+6 sema 测试 + 1 集成测试, 209 测试全部通过

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/sema/typeck.c

@@ -226,6 +226,14 @@ bool reorder_named_args(AstNode* node, Symbol* sym, int param_offset,
 
 void analyze_call_expr(AstNode* node, Scope* scope, ErrorList* errors, Arena* a) {
     Symbol* sym = scope_lookup(scope, node->as.call.name);
+    // 闭包调用: 变量类型为 TYPE_CLOSURE
+    if (sym && sym->kind == SYM_VARIABLE && sym->type == TYPE_CLOSURE) {
+        // 暂不做参数类型检查(MVP), 只分析参数表达式
+        for (size_t i = 0; i < node->as.call.arg_count; i++)
+            analyze_expr(node->as.call.args[i], scope, errors, a);
+        node->type.kind = TYPE_I64;  // 默认返回 i64(MVP 限制)
+        return;
+    }
     if (!sym || sym->kind != SYM_FUNCTION) {
         error_add(errors, "<sema>", node->loc.line, node->loc.col,
                   "未定义的函数 '%s'", node->as.call.name);
@@ -523,6 +531,53 @@ void analyze_method_call(AstNode* node, Scope* scope, ErrorList* errors, Arena*
         return NULL; \
     }
 
+// === lambda 表达式分析 ===
+static int lambda_counter = 0;
+
+void analyze_lambda(AstNode* node, Scope* scope, ErrorList* errors, Arena* a) {
+    lambda_counter++;
+    int name_len = snprintf(NULL, 0, "__lambda_%d", lambda_counter) + 1;
+    char* gen_name = arena_alloc_impl(a, name_len);
+    snprintf(gen_name, name_len, "__lambda_%d", lambda_counter);
+    node->as.lambda.generated_name = gen_name;
+
+    // 分析 lambda 体(参数作用域)
+    Scope* lambda_scope = scope_new(a, scope);
+    for (size_t i = 0; i < node->as.lambda.param_count; i++) {
+        AstNode* p = node->as.lambda.params[i];
+        scope_insert(lambda_scope, a, p->as.parameter.name, SYM_PARAMETER, p->as.parameter.type);
+    }
+    TypeKind saved_ret = current_return_type;
+    const char* saved_ret_sn = current_return_struct_name;
+    current_return_type = node->as.lambda.return_type;
+    current_return_struct_name = node->as.lambda.return_struct_type_name;
+    analyze_node(node->as.lambda.body, lambda_scope, errors, a);
+    current_return_type = saved_ret;
+    current_return_struct_name = saved_ret_sn;
+
+    // 创建顶层函数 AST 节点, 加入队列供 codegen 使用
+    AstNode* fn = ast_make_function(a, gen_name,
+        node->as.lambda.params, node->as.lambda.param_count,
+        node->as.lambda.return_type,
+        node->as.lambda.return_struct_type_name,
+        node->as.lambda.body, false, NULL, 0, node->loc);
+    if (lambda_count < 256)
+        lambda_queue[lambda_count++] = fn;
+
+    // 注册函数符号(支持递归调用自身)
+    TypeKind* pts = node->as.lambda.param_count > 0
+        ? arena_alloc_impl(a, node->as.lambda.param_count * sizeof(TypeKind)) : NULL;
+    for (size_t i = 0; i < node->as.lambda.param_count; i++)
+        pts[i] = node->as.lambda.params[i]->as.parameter.type;
+    scope_insert_function(scope, a, gen_name,
+        node->as.lambda.return_type,
+        node->as.lambda.return_struct_type_name,
+        pts, NULL, NULL, node->as.lambda.param_count, NULL, 0);
+
+    node->type.kind = TYPE_CLOSURE;
+}
+
+SEMA_HANDLER(analyze_lambda)
 SEMA_HANDLER(analyze_ident_expr)
 SEMA_HANDLER(analyze_unary_expr)
 SEMA_HANDLER(analyze_binary_expr)
@@ -572,6 +627,7 @@ void analyze_expr_init(void) {
     ast_dispatch_set(&sema_dispatch, AST_IF_STMT,      analyze_node_wrap);
     ast_dispatch_set(&sema_dispatch, AST_BLOCK,        analyze_node_wrap);
     ast_dispatch_set(&sema_dispatch, AST_LIST_COMP,    analyze_list_comp_wrap);
+    ast_dispatch_set(&sema_dispatch, AST_LAMBDA,      analyze_lambda_wrap);
 }
 
 void analyze_expr(AstNode* node, Scope* scope, ErrorList* errors, Arena* a) {