OutboxFeature loaded lotus_outbox_v1 once and then saved its own
in-memory list over the key on every change, so a second tab's save
dropped the first tab's unsent messages. persist() now re-reads the
stored list and applies the change (add/remove by txnId) to it.
Fixes#268
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5