Slight refinements to `._state.get_rt_dir()`

Per the `copilot` review,
https://github.com/goodboy/tractor/pull/406#pullrequestreview-3893270953

now we also,
- pass `exists_ok=True` to `.mkdir()` to avoid conc races.
- expose `appname: str` param for caller override.
- normalize `subdir` to avoid escaping the base rt-dir location.

tractor/_state.py

@@ -175,28 +175,50 @@ def current_ipc_ctx(
 
 def get_rt_dir(
     subdir: str|Path|None = None,
+    appname: str = 'tractor',
 ) -> Path:
     '''
     Return the user "runtime dir", the file-sys location where most
     userspace apps stick their IPC and cache related system
     util-files.
 
-    On linux we take use a `'${XDG_RUNTIME_DIR}/tractor/` subdir by
-    default but equivalents are mapped for each platform using
-    the lovely `platformdirs`.
+    On linux we use a `${XDG_RUNTIME_DIR}/tractor/` subdir by
+    default, but equivalents are mapped for each platform using
+    the lovely `platformdirs` lib.
 
     '''
     rt_dir: Path = Path(
         platformdirs.user_runtime_dir(
-            appname='tractor',
+            appname=appname,
         ),
     )
+
+    # Normalize and validate that `subdir` is a relative path
+    # without any parent-directory ("..") components, to prevent
+    # escaping the runtime directory.
     if subdir:
-        rt_dir: Path = rt_dir / subdir
+        subdir_path = (
+            subdir
+            if isinstance(subdir, Path)
+            else Path(subdir)
+        )
+        if subdir_path.is_absolute():
+            raise ValueError(
+                f'`subdir` must be a relative path!\n'
+                f'{subdir!r}\n'
+            )
+        if any(part == '..' for part in subdir_path.parts):
+            raise ValueError(
+                "`subdir` must not contain '..' components!\n"
+                f'{subdir!r}\n'
+            )
+
+        rt_dir: Path = rt_dir / subdir_path
 
     if not rt_dir.is_dir():
         rt_dir.mkdir(
             parents=True,
+            exist_ok=True,  # avoid `FileExistsError` from conc calls
         )
 
     return rt_dir