Bump UDS `listen()` backlog 1 -> 128 for multi-actor unreg

A backlog of 1 caused `ECONNREFUSED` when multiple
sub-actors simultaneously connect to deregister from
a remote-daemon registrar. Now matches the TCP
transport's default backlog (~128).

Also,
- add cross-ref comments between
  `_uds.close_listener()` and `async_main()`'s
  `parent_is_reg` deregistration path explaining
  the UDS socket-file lifecycle

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

tractor/ipc/_uds.py

@@ -300,7 +300,23 @@ async def start_listener(
     ):
         await sock.bind(str(bindpath))
 
-    sock.listen(1)
+    # NOTE, the backlog must be large enough to handle
+    # concurrent connection attempts during actor teardown.
+    # Previously this was `listen(1)` which caused
+    # deregistration failures in the remote-daemon registrar
+    # case: when multiple sub-actors simultaneously try to
+    # connect to deregister, a backlog of 1 overflows and
+    # connections get ECONNREFUSED. This matches the TCP
+    # transport which uses `trio.open_tcp_listeners()` with
+    # a default backlog of ~128.
+    #
+    # For details see the `close_listener()` below which
+    # `os.unlink()`s the socket file on teardown — meaning
+    # any NEW connection attempts after that point will fail
+    # with `FileNotFoundError` regardless of backlog size.
+    # The backlog only matters while the listener is alive
+    # and accepting.
+    sock.listen(128)
     log.info(
         f'Listening on UDS socket\n'
         f'[>\n'
@@ -316,6 +332,16 @@ def close_listener(
     '''
     Close and remove the listening unix socket's path.
 
+    NOTE, the `os.unlink()` here removes the socket file from
+    the filesystem immediately, which means any subsequent
+    connection attempts (e.g. sub-actors trying to deregister
+    with a registrar whose listener is tearing down) will fail
+    with `FileNotFoundError`. For the local-registrar case
+    (parent IS the registrar), `_runtime.async_main()` works
+    around this by reusing the existing `_parent_chan` instead
+    of opening a new connection; see the `parent_is_reg` logic
+    in the deregistration path.
+
     '''
     lstnr.socket.close()
     os.unlink(addr.sockpath)

tractor/runtime/_runtime.py

@@ -1848,11 +1848,17 @@ async def async_main(
             failed_unreg: bool = False
             rent_chan: Channel|None = actor._parent_chan
 
-            # XXX check if the parent IS the registrar so we can
-            # reuse the existing channel (avoids opening a new
-            # connection which fails when the listener socket is
-            # already closed, e.g. UDS transport unlinks the socket
-            # file during teardown).
+            # XXX check if the parent IS the registrar so we
+            # can reuse the existing `_parent_chan` (avoids
+            # opening a new connection which fails when the
+            # listener socket is already closed, e.g. UDS
+            # transport `os.unlink()`s the socket file during
+            # teardown).
+            #
+            # See `ipc._uds.close_listener()` for details on
+            # the UDS socket-file lifecycle and why this
+            # optimization is necessary for the local-registrar
+            # case.
             parent_is_reg: bool = False
             if (
                 rent_chan is not None