Fix `mk_maddr()` crash on absolute UDS paths

Strip leading `/` from `filepath` before building
the `/unix/{path}` multiaddr string; OW absolute
sockpaths like `/run/user/1000/tractor/foo.sock`
produce `/unix//run/..` which `py-multiaddr`
rejects as "empty protocol path".

Woops, missed this in the initial `mk_maddr()` impl
bc the unit tests only used relative `filedir` values
(which was even noted in a comment..). The bug only
surfaces when the `.maddr` property on `UDSTransport`
is hit during logging/repr with real runtime addrs.

Found-via: cross-suite `pytest tests/ipc/ tests/msg/`
where `tpt_proto='uds'` leaks into msg tests

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

tractor/discovery/_multiaddr.py

@@ -78,8 +78,13 @@ def mk_maddr(
         case 'uds':
             filedir, filename = addr.unwrap()
             filepath = Path(filedir) / filename
+            # NOTE, strip any leading `/` to avoid
+            # double-slash `/unix//run/..` which the
+            # multiaddr parser rejects as "empty
+            # protocol path".
+            fpath_str: str = str(filepath).lstrip('/')
             return Multiaddr(
-                f'/{maddr_proto}/{filepath}'
+                f'/{maddr_proto}/{fpath_str}'
             )