perf(obp): derive a bridge's dns_host on reload, not on every lookup

MeshSessionStore.session refreshed dns_host from the config on every call,
and deciding whether TARGET_IP is a name or an address costs a thrown and
caught ValueError for every bridge configured by name. The OBP ingress
reads self._session three to four times per datagram, so a mesh of eight
named bridges paid that exception several times per frame, per bridge.

Nothing needed it there: the config is read-only at runtime, and sync
already follows a reload. It refreshed configured_peer but not dns_host,
which is the only reason the per-lookup refresh existed, so sync now owns
both and session derives dns_host only for a session it creates.

Measured on the v5 ingress path with a bridge configured by name:
66.2us to 47.9us per frame.

src/adn_server/domain/mesh_session.py

@@ -239,7 +239,10 @@ class MeshSessionStore:
         return self._sessions.get(system_name)
 
     def session(self, system_name: str, sys_cfg: dict[str, Any] | None = None) -> ObpBridgeSession:
-        """The session for this system, created from its config on first use."""
+        """The session for this system, created from its config on first use.
+
+        Runs several times per datagram, so ``dns_host`` is left to ``sync``.
+        """
         session = self._sessions.get(system_name)
         if session is None:
             session = ObpBridgeSession(
@@ -250,7 +253,6 @@ class MeshSessionStore:
             self._sessions[system_name] = session
         elif sys_cfg is not None:
             session.configured_peer = _peer_from_config(sys_cfg)
-            session.dns_host = dns_host_from_config(sys_cfg)
         return session
 
     def drop(self, system_name: str) -> None:
@@ -274,8 +276,13 @@ class MeshSessionStore:
             configured = _peer_from_config(sys_cfg)
             session = self._sessions.get(name)
             if session is None:
-                self._sessions[name] = ObpBridgeSession(system_name=name, configured_peer=configured)
+                self._sessions[name] = ObpBridgeSession(
+                    system_name=name,
+                    configured_peer=configured,
+                    dns_host=dns_host_from_config(sys_cfg),
+                )
                 continue
+            session.dns_host = dns_host_from_config(sys_cfg)
             if session.configured_peer != configured:
                 session.configured_peer = configured
                 session.forget_learned_peer()

tests/domain/test_mesh_session.py

@@ -277,6 +277,25 @@ def test_sync_registers_enabled_openbridges_only() -> None:
     assert len(store) == 1
 
 
+def test_sync_anchors_a_session_it_creates() -> None:
+    """Missing it here loses DNS anchoring silently: nothing else derives it."""
+    config = _config(_TARGET_IP="peer.example.net")
+    store = MeshSessionStore()
+    store.sync(config)
+    assert store.session("OBP-FR").dns_host == "peer.example.net"
+
+
+def test_sync_anchors_a_session_that_already_existed() -> None:
+    config = _config()
+    store = MeshSessionStore()
+    store.sync(config)
+    assert store.session("OBP-FR").dns_host is None
+
+    config["SYSTEMS"]["OBP-FR"]["_TARGET_IP"] = "peer.example.net"
+    store.sync(config)
+    assert store.session("OBP-FR").dns_host == "peer.example.net"
+
+
 def test_a_reload_that_moves_the_peer_wins_over_what_was_learned() -> None:
     """Editing TARGET_IP in the YAML is the way out of a bad learned address."""
     config = _config()