Fleet: get_fleet (read, handles not-in-fleet); guarded invite_to_fleet and
update_fleet. UI: guarded set_waypoint and open_window (market/info/contract).
All writes go through write-safety.
Closes#27, #28. 128 tests, coverage 89%.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
