feat(deckies): live service add/remove without full redeploy

decnet.engine.services_live exposes add_service / remove_service for
both fleet and topology decky scopes.  The host's _compose() wrapper
already supported per-service targeting (up --no-deps -d <svc>,
stop, rm -f); what was missing was the orchestration around it:

* add: validate against decnet.services.registry (rejects unknown +
  fleet_singleton); persist the new services list; re-render the
  per-scope compose file (so future redeploys reflect the change);
  run docker compose up -d --no-deps --build <decky>-<svc>.
* remove: stop + rm -f the service container; persist; re-render
  compose so a future up -d doesn't bring it back.

Both publish decky.<name>.service.added / .removed on the bus, with
the post-mutation services list.  Topic constants added to
decnet.bus.topics; the matching wiki entry in wiki-checkout/Service-Bus.md
ships in a separate commit on the wiki repo (wiki-checkout/ is gitignored).

Four new admin endpoints:

* POST/DELETE /api/v1/deckies/{name}/services{,/svc}
* POST/DELETE /api/v1/topologies/{id}/deckies/{name}/services{,/svc}

ServiceMutationError messages are mapped at the API boundary to 404
(decky/topology missing), 409 (idempotency violation), 422 (unknown
or fleet_singleton service).

decnet/bus/topics.py

@@ -83,6 +83,13 @@ DECKY_MUTATE_REQUEST = "mutate_request"
 # syslog sidechannel too) to interleave substrate-change markers into
 # attacker traversals.
 DECKY_MUTATION = "mutation"
+# Per-service add/remove on a deployed decky (live; no full redeploy).
+# Payload carries ``decky_name``, ``service_name``, optional
+# ``topology_id``, and ``services`` (the post-mutation list).  Consumers
+# that watch substrate shape (correlator, dashboard, profiler) reconcile
+# off these without waiting for the next decnet-state.json snapshot.
+DECKY_SERVICE_ADDED = "service.added"
+DECKY_SERVICE_REMOVED = "service.removed"
 
 # Attacker event types (second token under the ``attacker`` root).  First
 # sighting, session boundary transitions, and score-threshold crossings