Co-op offline with an AI partner

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lang/en.lang

@@ -662,6 +662,8 @@ net.coop_create = Co-op campaign
 net.coop_editor = Editor campaign
 net.coop_builtin = Built-in campaign
 net.coop_league = League against the Lightwatch
+net.coop_solo = AI partner
+net.coop_solo_started = Co-op without the server: your AI partner builds its own dungeon, follows your attack flag and helps out with gold.
 net.coop_editor_mission = Editor campaign {0}: mission {1} ({2})
 net.coop_editor_none = No mission of the editor campaign fits co-op (map at least 72x72, a goal besides rival masters)
 net.join = Join
@@ -709,6 +711,7 @@ msg.wave_on_rival1 = Wave {0}: {1} heroes march on the green master.
 msg.wave_on_rival2 = Wave {0}: {1} heroes march on the blue master.
 msg.wave_warn_on_master = Scouts report: hero wave {0} comes in 30 seconds – it heads for master {1}.
 msg.wave_on_master = Wave {0}: {1} heroes march on master {2}.
+msg.partner_out = Your AI partner's heart has fallen – fight on alone!
 hud.rival1 = Green
 hud.rival2 = Blue
 msg.creature_freed = A {0} breaks its chains and joins you!