fix(switch): case 标签支持裸名（按值引用）并给出常量标签 fail-fast

- 裸名在 match 里是捕获模式（MatchAs），switch 里对齐 C 的 `case NAME:` 语义，
  三端解析均归一为 MatchValue(Name)；`_` 仍作通配（等价 default）
- includes/ast/parser.vp、lib/vast/Parser.py、lib/vast_c/vast_c.c 同步实现
- HandlesMatch：case 标签求值非编译期常量时 fail-fast（原先会因取 .constant 崩成
  AttributeError），报文明确“case 标签必须是编译期常量”

lib/vast_c/vast_c.c

@@ -6456,6 +6456,17 @@ static int SwitchPatAllowed(PyObject *Pat) {
         || strcmp(N, "MatchClass") == 0) {
         return 1;
     }
+    /* 通配 `case _`（MatchAs 无子模式）等价 default */
+    if (strcmp(N, "MatchAs") == 0) {
+        PyObject *Inner = PyObject_GetAttrString(Pat, "pattern");
+        if (Inner == NULL) {
+            PyErr_Clear();
+            return 0;
+        }
+        int IsWild = (Inner == Py_None);
+        Py_DECREF(Inner);
+        return IsWild;
+    }
     if (strcmp(N, "MatchOr") == 0) {
         PyObject *Pats = PyObject_GetAttrString(Pat, "patterns");
         if (Pats == NULL) {
@@ -6479,6 +6490,52 @@ static int SwitchPatAllowed(PyObject *Pat) {
     return 0;
 }
 
+/* switch 的 case 标签修正：裸名按**值引用**处理
+ *
+ * match 里裸名是捕获模式（MatchAs，pattern=None），switch 里对齐 C 的 `case NAME:`
+ * 语义 —— 归一为 MatchValue(Name(name, Load))；`_`（name=None）仍是通配，原样保留。 */
+static PyObject *SwitchLabelFromPattern(Parser *P, PyObject *Pat) {
+    if (Pat == NULL || strcmp(Py_TYPE(Pat)->tp_name, "MatchAs") != 0) {
+        return Pat;
+    }
+    PyObject *Inner = PyObject_GetAttrString(Pat, "pattern");
+    PyObject *NameStr = PyObject_GetAttrString(Pat, "name");
+    if (Inner == NULL || NameStr == NULL) {
+        PyErr_Clear();
+        Py_XDECREF(Inner);
+        Py_XDECREF(NameStr);
+        return Pat;
+    }
+    int IsBareName = (Inner == Py_None && NameStr != Py_None);
+    Py_DECREF(Inner);
+    if (!IsBareName) {
+        Py_DECREF(NameStr);
+        return Pat;
+    }
+    PyObject *Load = MakeCtx(P, "Load");
+    if (Load == NULL) {
+        Py_DECREF(NameStr);
+        return Pat;
+    }
+    PyObject *NameArgv[2] = { NameStr, Load };
+    PyObject *NameNode = NewNode(P, "Name", NameArgv, 2);
+    Py_DECREF(Load);
+    if (NameNode == NULL) {
+        Py_DECREF(NameStr);
+        return Pat;
+    }
+    /* Name 文本已在 NameNode 内自持，可安全释放 */
+    Py_DECREF(NameStr);
+    PyObject *MvArgv[1] = { NameNode };
+    PyObject *Mv = NewNode(P, "MatchValue", MvArgv, 1);
+    Py_DECREF(NameNode);
+    if (Mv == NULL) {
+        return Pat;
+    }
+    Py_DECREF(Pat);
+    return Mv;
+}
+
 /* switch 语句（C switch 语义）：脱糖为 is_switch=True 的 Match
  *
  *   switch <expr>:
@@ -6523,6 +6580,7 @@ static PyObject *ParseSwitch(Parser *P) {
                 Py_DECREF(Subject);
                 return NULL;
             }
+            Pattern = SwitchLabelFromPattern(P, Pattern);
             if (!SwitchPatAllowed(Pattern)) {
                 SyntaxErrorAt(P, CaseStart,
                     "switch case label must be a constant or enum variant");