Make run activity marker writes atomic

[yvette-carlisle]

Summary

• write .decodex-run-activity through an atomic temp-file rename
• keep account marker data stable across high-frequency activity refreshes
• add regression coverage for account preservation after marker refresh

Validation

• cargo test -p decodex state::tests::run_activity_marker_round_trips_marker_surfaces -- --nocapture
• cargo test -p decodex operator_dashboard_websocket_sends_current_run_activity_on_connect -- --nocapture
• cargo test -p decodex operator_dashboard_run_activity_event_summarizes_active_runs -- --nocapture
• cargo make check-rust
• cargo make check-vstyle-rust
• git diff --check