Use `_tpt_proto_to_maddr` lookup in `mk_maddr()`

Address Copilot review: the mapping table was
defined but never referenced. Now `mk_maddr()`
resolves `proto_key` -> maddr protocol name via
the table and rejects unknown keys upfront.

Also add missing `Path` import to the `multiaddr`
usage snippet.

Review: PR #429 (Copilot)
https://github.com/goodboy/tractor/pull/429#pullrequestreview-4010456884

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

snippets/multiaddr_ex.py

@@ -1,4 +1,6 @@
+from pathlib import Path
 from multiaddr import Multiaddr
+
 # construct from a string
 m1 = Multiaddr("/ip4/127.0.0.1/udp/1234")
 m2 =  Multiaddr("/unix/run/user/1000/sway-ipc.1000.1557.sock")

tractor/discovery/_multiaddr.py

@@ -50,7 +50,14 @@ def mk_maddr(
     multiaddr-spec-compliant protocol path.
 
     '''
-    match addr.proto_key:
+    proto_key: str = addr.proto_key
+    maddr_proto: str|None = _tpt_proto_to_maddr.get(proto_key)
+    if maddr_proto is None:
+        raise ValueError(
+            f'Unsupported proto_key: {proto_key!r}'
+        )
+
+    match proto_key:
         case 'tcp':
             host, port = addr.unwrap()
             ip = ipaddress.ip_address(host)
@@ -59,17 +66,12 @@ def mk_maddr(
                 else 'ip6'
             )
             return Multiaddr(
-                f'/{net_proto}/{host}/tcp/{port}'
+                f'/{net_proto}/{host}/{maddr_proto}/{port}'
             )
 
         case 'uds':
             filedir, filename = addr.unwrap()
             filepath = Path(filedir) / filename
             return Multiaddr(
-                f'/unix/{filepath}'
-            )
-
-        case _:
-            raise ValueError(
-                f'Unsupported proto_key: {addr.proto_key!r}'
+                f'/{maddr_proto}/{filepath}'
             )