fix(narrow): BuildPipeline.ensure_dir 消除隐式窄化（int(path_len) / char(saved)）

buf[i] 元素为 i8，saved 由 int 改为 char 并用 char(ch) 显式收窄；plen 由 sizet 显式 int(path_len) 收窄

App/lib/Phase/BuildPipeline.vp

@@ -290,11 +290,11 @@ def ensure_dir(path: str) -> int:
 
     # Temporarily truncate at each separator and create every directory level.
     # CreateDirectoryA returns 0 when the dir exists (failure); ignore it.
-    plen: int = path_len
+    plen: int = int(path_len)
     for i in range(plen):
         ch: int = buf[i]
         if ch == 47 or ch == 92:  # '/' = 47, '\\' = 92
-            saved: int = ch
+            saved: char = char(ch)
             buf[i] = '\0'
             win32.win32file.CreateDirectoryA(buf, None)
             buf[i] = saved