fix(bus): topic segments can't contain dots — service.added → service_added

Bus topic segments are NATS-style tokens and the validator at
bus/topics.py:402 rejects '.', '*', '>', whitespace.  My W3 constants
'service.added' / 'service.removed' tripped this on every live
add/remove call:

  ValueError: topic segment 'service.added' may not contain '.', ...

Renamed both to underscore form: DECKY_SERVICE_ADDED = 'service_added'.
Aligned the SSE forwarder's name mapping (decky.<name>.service_added →
SSE event 'decky.service_added') and the frontend's
useTopologyStream listener + MazeNET.tsx event handler.  Also updated
the wiki entry with a note about the underscore.

decnet/web/router/topology/api_events.py

@@ -177,10 +177,15 @@ def _sse_name_for(topic: str) -> str:
 
     ``topology.<id>.mutation.applied``        → ``mutation.applied``
     ``topology.<id>.status``                  → ``status``
-    ``decky.<name>.service.added``            → ``decky.service.added``
-    ``decky.<name>.service.removed``          → ``decky.service.removed``
+    ``decky.<name>.service_added``            → ``decky.service_added``
+    ``decky.<name>.service_removed``          → ``decky.service_removed``
     Anything else is passed through unchanged so future topic families
     don't silently collapse onto a generic bucket.
+
+    Bus topic segments are NATS-style tokens — no dots inside a segment
+    — which is why the leaf is ``service_added`` (underscore) here and
+    on the wire, not ``service.added``.  The frontend's
+    ``useTopologyStream`` listens on the underscore form too.
     """
     parts = topic.split(".", 2)
     if len(parts) < 3: