Tidy up `open_actor_cluster()`, use `.aid`

Update cluster spawning logic to use `.aid` instead of
`.uid` and improve code clarity, typing, style throughout.

Deats,
- change `uid` -> `aid` and use `.aid.name` for cluster
  naming prefix
- add doc-string explaining cluster as actor-based worker
  pool returning `dict[str, Portal]` table.
- improve error msg formatting for name/count mismatch
- change comparison: `not len(names) == count` ->
  `len(names) != count`.
- use remove no-whitespace union style.
- enhance assert msg for portal-count check
- use multiline style where appropes.

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

tractor/_clustering.py

@@ -25,17 +25,21 @@ from contextlib import (
 from multiprocessing import cpu_count
 from typing import (
     AsyncGenerator,
+    TYPE_CHECKING,
 )
 
 import trio
 import tractor
 
+if TYPE_CHECKING:
+    from tractor.msg import Aid
+
 
 @acm
 async def open_actor_cluster(
     modules: list[str],
     count: int = cpu_count(),
-    names: list[str] | None = None,
+    names: list[str]|None = None,
     hard_kill: bool = False,
 
     # passed through verbatim to ``open_root_actor()``
@@ -45,15 +49,26 @@ async def open_actor_cluster(
     dict[str, tractor.Portal],
     None,
 ]:
+    '''
+    Open an "actor cluster", much like a "process worker pool" but where
+    each primitive is a full `tractor.Actor` allocated as a batch and
+    mapped via a `dict[str, Portal]` table returned to the caller.
 
+    '''
     portals: dict[str, tractor.Portal] = {}
 
     if not names:
-        names = [f'worker_{i}' for i in range(count)]
+        names: list[str] = [
+            f'worker_{i}'
+            for i in range(count)
+        ]
 
-    if not len(names) == count:
+    if len(names) != count:
         raise ValueError(
-            'Number of names is {len(names)} but count it {count}')
+            f'Number of subactor names != count ??\n'
+            f'len(name) = {len(names)!r}\n'
+            f'count = {count!r}\n'
+        )
 
     async with (
         # tractor.trionics.collapse_eg(),
@@ -66,18 +81,18 @@ async def open_actor_cluster(
             trio.open_nursery() as tn,
             tractor.trionics.maybe_raise_from_masking_exc()
         ):
-            uid = tractor.current_actor().uid
-
+            aid: Aid = tractor.current_actor().aid
             async def _start(name: str) -> None:
-                name = f'{uid[0]}.{name}'
                 portals[name] = await an.start_actor(
                     enable_modules=modules,
-                    name=name,
+                    name=f'{aid.name}.{name}',
                 )
 
             for name in names:
                 tn.start_soon(_start, name)
 
-        assert len(portals) == count
+        assert len(portals) == count, 'Portal-count mismatch?\n'
         yield portals
-        await an.cancel(hard_kill=hard_kill)
+        await an.cancel(
+            hard_kill=hard_kill
+        )