feat: use 语句 — use module::item 导入模块符号

修复审查 #2: use 语句从 skippable TODO 实现为完整符号导入
  use math_mod::add → 加载模块, 匹配 pub 函数/结构体/枚举, 零前缀导入
新增 37_use.l 集成测试

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

src/parser/parser.c

@@ -534,10 +534,47 @@ AstNode* parse(Arena* a, const Token* tokens, size_t count,
                 enums[enum_count++] = sub->as.program.enums[i];
             }
         } else if (peek(&p)->kind == TOK_USE) {
-            /* TODO: use 语句待实现符号导入 */ ;
-            advance(&p);
-            while (peek(&p)->kind != TOK_SEMICOLON && peek(&p)->kind != TOK_EOF) advance(&p);
-            if (peek(&p)->kind == TOK_SEMICOLON) advance(&p);
+            advance(&p);  // 跳过 use
+            const Token* mod_tok = expect(&p, TOK_IDENT, error, "use 后应为模块名");
+            if (!mod_tok) return NULL;
+            if (!expect(&p, TOK_COLON_COLON, error, "缺少 '::'")) return NULL;
+            const Token* item_tok = expect(&p, TOK_IDENT, error, "use 后应为项目名");
+            if (!item_tok) return NULL;
+            if (!expect(&p, TOK_SEMICOLON, error, "缺少 ';'")) return NULL;
+            const char* mod_name = arena_strdup_impl(p.arena, mod_tok->start, mod_tok->length);
+            const char* item_name = arena_strdup_impl(p.arena, item_tok->start, item_tok->length);
+            AstNode* sub = load_module(a, filename, mod_name, error);
+            if (!sub) return NULL;
+            // 在模块中搜索指定项目（函数/结构体/枚举）
+            bool found = false;
+            for (size_t i = 0; i < sub->as.program.fn_count; i++) {
+                AstNode* fn = sub->as.program.functions[i];
+                if (fn->as.function.is_pub && strcmp(fn->as.function.name, item_name) == 0) {
+                    if (fn_count >= 256) { error->message = "函数过多"; error->filename = p.filename; return NULL; }
+                    functions[fn_count++] = fn;
+                    found = true; break;
+                }
+            }
+            if (!found) {
+                for (size_t i = 0; i < sub->as.program.struct_count; i++) {
+                    AstNode* sd = sub->as.program.structs[i];
+                    if (strcmp(sd->as.struct_decl.name, item_name) == 0) {
+                        if (struct_count >= 64) break;
+                        structs[struct_count++] = sd;
+                        found = true; break;
+                    }
+                }
+            }
+            if (!found) {
+                for (size_t i = 0; i < sub->as.program.enum_count; i++) {
+                    AstNode* ed = sub->as.program.enums[i];
+                    if (strcmp(ed->as.enum_decl.name, item_name) == 0) {
+                        if (enum_count >= 64) break;
+                        enums[enum_count++] = ed;
+                        found = true; break;
+                    }
+                }
+            }
         } else if (peek(&p)->kind == TOK_FN) {
             if (fn_count >= 256) { error->message = "函数过多 (最多256)"; error->filename = p.filename; error->line = peek(&p)->line; error->col = peek(&p)->col; return NULL; }
             functions[fn_count++] = parse_function(&p, is_pub, error);