Fix `TIPC` two-host service example

A registrar root does not register itself in its own actor-name
registry, so host B could never discover the advertised `host_a`.
Boot that service as a child actor under the `TIPC` registrar
instead.

Import and pass the enabled `echo` callable to `.open_context()`;
the prior module-path string could not produce a `NamespacePath`.

Review: PR #493 (copilot-pull-request-reviewer[bot],goodboy)
https://github.com/goodboy/tractor/pull/493

(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))

examples/multihost/tipc_cluster/host_a_srv.py

@@ -47,14 +47,16 @@ async def main() -> None:
     reg: TIPCAddress = TIPCAddress.get_root()
     print(f'host A publishing {reg}')
 
-    async with tractor.open_root_actor(
+    async with tractor.open_nursery(
-        name='host_a',
         enable_transports=['tipc'],
         registry_addrs=[reg.unwrap()],
-        enable_modules=[__name__],
+    ) as an:
-    ):
+        await an.start_actor(
+            'host_a',
+            enable_modules=[__name__],
+        )
         print(
-            'registrar up — `tipc nametable show` on EITHER host\n'
+            'host_a up — `tipc nametable show` on EITHER host\n'
             'should now list this service. ctrl-c to stop.'
         )
         await trio.sleep_forever()

examples/multihost/tipc_cluster/host_b_client.py

@@ -16,6 +16,7 @@ from __future__ import annotations
 
 import trio
 import tractor
+from host_a_srv import echo
 from tractor.ipc._tipc import (
     TIPCAddress,
     is_tipc_available,
@@ -41,7 +42,7 @@ async def main() -> None:
 
             async with (
                 ptl.open_context(
-                    'host_a_srv:echo',
+                    echo,
                 ) as (ctx, _),
                 ctx.open_stream() as stream,
             ):