按源验证API百分号文件模式

源依据：LinuxCNC src/emc/rs274ngc/rs274ngc_pre.cc 的 Interp::open() 在首个非空行是 % 时启用 percent-file mode，src/emc/rs274ngc/interp_read.cc read_text() 在匹配 % 处结束读取并忽略后续文本。

结论：native API source-linked smoke 增加 CNC_SIM_RS274_FILE_MODE 下的百分号文件断言，确认 cnc_sim API 文件模式走 LinuxCNC open/read 语义而非逐行桥接或 smoke fallback。

检查：./test-linuxcnc-api-native.sh；./test-native.sh；./test-linuxcnc-source-link.sh。

core/tests/cnc_sim_api_linuxcnc_rs274_smoke.cpp

@@ -303,6 +303,45 @@ int main() {
         }
     }
 
+    {
+        std::vector<CncSimEvent> percent_events;
+        CncSimHandle *percent_sim = cnc_sim_create();
+        cnc_sim_set_event_callback(percent_sim, collect_event, &percent_events);
+        ScopedEnv file_mode_env("CNC_SIM_RS274_FILE_MODE", "1");
+        const char percent_program[] =
+            "%\n"
+            "G21 G90\n"
+            "F100\n"
+            "G1 X1\n"
+            "%\n"
+            "G1 X99\n";
+        bool percent_ok = true;
+        percent_ok &= expect(cnc_sim_parse_program(percent_sim,
+                                                   percent_program,
+                                                   sizeof(percent_program) - 1) == 0,
+                             cnc_sim_last_error(percent_sim));
+        std::vector<int> feed_lines;
+        std::vector<double> feed_x;
+        for (const auto &event : percent_events) {
+            if (event.type == CNC_SIM_EVENT_LINEAR_FEED) {
+                feed_lines.push_back(event.line);
+                feed_x.push_back(event.end.x);
+            }
+        }
+        // LinuxCNC source basis: rs274ngc_pre.cc open() enables percent-file
+        // mode when the first nonblank line is '%'; interp_read.cc read_text()
+        // stops at the matching '%', so later program text is ignored.
+        percent_ok &= expect(feed_lines.size() == 1 &&
+                                 feed_lines[0] == 4 &&
+                                 feed_x.size() == 1 &&
+                                 near(feed_x[0], 1.0),
+                             "expected LinuxCNC percent-file mode to ignore trailing text");
+        cnc_sim_destroy(percent_sim);
+        if (!percent_ok) {
+            return 1;
+        }
+    }
+
     const char config[] =
         "{\"backend\":\"linuxcnc-rs274\",\"rtcp\":{\"enabled\":true,\"toolLength\":100,\"toolLengths\":{\"7\":125}}}";
     const char program[] =