feat(coro): M7——try/finally、异常消息、自定义异常类、帧池复用

- try/finally（无 except）：正常/异常路径都执行 fbody，异常时 finally 后
  重新传播（done+br exit）；finally 内暂不支持 await
- 异常消息：帧加 msg 槽（i8*，index 4），raise X(msg) 存消息，await 传播，
  coro_exception_msg 查询
- raise 支持自定义异常类（exception_registry 码），多 except 匹配子类码
- 帧池复用：per-type free-list 全局池，create 弹池/空则 malloc，coro_destroy
  回池（帧头部存 next），避免反复 malloc/free
- 修复 ast.walk 传 list 崩溃（finalbody/For body）
- CoroTest 验证：finally 执行、MyError exc=100 msg=boom msg、3 次循环池复用

Test/CoroTest/App/main.vp

@@ -4,35 +4,38 @@ import stdio
 import coro
 
 
+class MyError(Exception):
+    pass
+
+
 async def leaf(x: int) -> int:
     return x + 1
 
 
-async def boom(kind: int) -> int:
-    if kind == 1:
-        raise ValueError
-    raise KeyError
+async def boom() -> int:
+    raise MyError("boom msg")
     return 0
 
 
-async def task(kind: int) -> int:
-    caught: int = 0
+async def task() -> int:
+    fin: int = 0
     try:
-        await boom(kind)
-    except ValueError as e:
-        caught = 1
-    except KeyError:
-        caught = 2
-    return caught
+        fin = 1
+        await boom()
+    finally:
+        fin = fin + 1
+        stdio.printf("finally ran, fin = %d\n", fin)
+    return 99
 
 
 def main() -> t.CInt:
-    a: coro.Coroutine = task(1)
-    b: coro.Coroutine = task(2)
-    n: t.CInt = coro.coro_gather(a, b)
-    r1: t.CInt = coro.coro_run(a)
-    r2: t.CInt = coro.coro_run(b)
-    stdio.printf("gather = %d, r1 = %d, r2 = %d\n", n, r1, r2)
-    coro.coro_destroy(a)
-    coro.coro_destroy(b)
+    n: t.CInt = 0
+    for i in range(3):
+        tt: coro.Coroutine = task()
+        r: t.CInt = coro.coro_run(tt)
+        e: t.CInt = coro.coro_exception(tt)
+        m: t.CPtr = coro.coro_exception_msg(tt)
+        stdio.printf("[%d] r = %d, exc = %d, msg = %s\n", i, r, e, m)
+        coro.coro_destroy(tt)
+        n = n + 1
     return 0