feat(switch): 新增 switch 语句并禁止 match 内 fallthrough

- MatchGenMixin：case 体按 node.is_switch 置 _in_switch_case；_has_no_break 只认 FallThrough 节点（去掉 c.NoBreak() 兼容路径）
- StmtGenDeclMixin：translate_fallthrough 只在 switch 的 case 体内合法，match/switch 之外 fail-fast
- Translator：_in_match_case 更名 _in_switch_case（语义 = 当前 case 体属于 switch）
- FuncGenDef / FuncGenMixin：函数体 / 嵌套函数 / lambda 入口隔离并恢复 _in_switch_case
- 因 match 改为硬关键字，重命名同名局部变量 match -> Matched（StubMergerStore / ImportsGenMixin / ExprGenAttr / CallGenBase）

App/lib/Stub/StubMerger/StubMergerStore.vp

@@ -554,14 +554,14 @@ def LookupSha1ByAlias(from_imports: str, alias: str) -> str:
         local_len: sizet = colon - ci
         if colon < total_len and from_imports[colon] == ':' and local_len == alias_len:
             # 比较 local_name == alias
-            match: int = 1
+            Matched: int = 1
             li: sizet = 0
             while li < alias_len:
                 if from_imports[ci + li] != alias[li]:
-                    match = 0
+                    Matched = 0
                     break
                 li += 1
-            if match == 1:
+            if Matched == 1:
                 # 提取 module（colon+1 到下一个 ':' 或空格）
                 m_start: sizet = colon + 1
                 m_end: sizet = m_start
@@ -609,14 +609,14 @@ def LookupModNameByAlias(from_imports: str, alias: str) -> str:
         local_len: sizet = colon - ci
         if colon < total_len and from_imports[colon] == ':' and local_len == alias_len:
             # 比较 local_name == alias
-            match: int = 1
+            Matched: int = 1
             li: sizet = 0
             while li < alias_len:
                 if from_imports[ci + li] != alias[li]:
-                    match = 0
+                    Matched = 0
                     break
                 li += 1
-            if match == 1:
+            if Matched == 1:
                 # 提取 module（colon+1 到下一个 ':' 或空格）
                 m_start: sizet = colon + 1
                 m_end: sizet = m_start