Mv `._clustering` -> `.experimental._cluster`

Move clustering API to experimental subpkg since it's not
yet stabilized for public use.

Deats,
- rename mod as `._clustering.py` -> ._cluster.py`.
- drop `open_actor_cluster` from `tractor.__init__`
- add `open_actor_cluster` to `tractor.experimental.__init__`
- update imports in rest of repo.
- fix typo in `tractor.__init__` doc-string: "structurred"
  -> "structured"
- use multiline import style for `._pubsub` in experimental
  `__init__`

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

examples/quick_cluster.py

@@ -19,7 +19,7 @@ async def main():
     # look at this hip new syntax!
     async with (
 
-        tractor.open_actor_cluster(
+        tractor.experimental.open_actor_cluster(
             modules=[__name__]
         ) as portal_map,
 

tests/test_clustering.py

@@ -3,7 +3,7 @@ import itertools
 import pytest
 import trio
 import tractor
-from tractor import open_actor_cluster
+from tractor.experimental import open_actor_cluster
 from tractor.trionics import gather_contexts
 from tractor._testing import tractor_test
 

tractor/__init__.py

@@ -15,13 +15,9 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 """
-tractor: structured concurrent ``trio``-"actors".
+tractor: distributed structurred concurrency
 
 """
-
-from ._clustering import (
-    open_actor_cluster as open_actor_cluster,
-)
 from ._context import (
     Context as Context,  # the type
     context as context,  # a func-decorator

tractor/experimental/__init__.py

@@ -21,7 +21,12 @@ built-ins.
 This is a staging area for ``tractor.builtin``.
 
 '''
-from ._pubsub import pub as msgpub
+from ._cluster import (
+    open_actor_cluster as open_actor_cluster,
+)
+from ._pubsub import (
+    pub as msgpub,
+)
 
 
 __all__ = [