落地 12.2 二十个用例目录：toml + .st + 期望表。

- tests/cases/ 01~20：13 正例 / 6 负例 / 1 运行时故障
- 负例冻结报错类别：非 GVL 写全局、同名全局、io.var 未声明、FUNCTION 写全局、未声明实例、循环调用
- tests/README 补充用例清单与点亮节奏

tests/cases/14_function_write_global/EXPECTED.md

@@ -0,0 +1,4 @@
+# 期望：编译错误
+
+- `FUNCTION` 内写全局（此处经 `VAR_EXTERNAL` 写 `G1`）
+- 报错类别：FUNCTION 禁止写全局

tests/cases/14_function_write_global/globals.st

@@ -0,0 +1,3 @@
+VAR_GLOBAL
+    G1 : INT;
+END_VAR

tests/cases/14_function_write_global/main.st

@@ -0,0 +1,9 @@
+FUNCTION Bad : INT
+VAR_EXTERNAL
+    G1 : INT;
+END_VAR
+    G1 := 1;
+END_FUNCTION
+
+PROGRAM MAIN
+END_PROGRAM

tests/cases/14_function_write_global/project.toml

@@ -0,0 +1,11 @@
+[project]
+name = "case14"
+entry = "program MAIN"
+cycle_limit = 1000
+dt_ms = 10
+
+[files]
+st = ["globals.st", "main.st"]
+
+[gvl]
+file = "globals.st"