Under the cargo test fallback (threads in one process), tests that touch
the process-global scanner cycle recovery status or the global usage-save
metrics raced each other and failed randomly in full-suite runs.
Mark all touchers with #[serial] per docs/testing/README.md:
- 22 tests reading or writing scanner_cycle_recovery_status() via
load_scanner_cycle_state_for_startup / reset_scanner_cycle_recovery
- 24 tests mutating global_metrics() usage-save counters via
store_data_usage_in_backend*, which raced the existing serial
test_deferred_usage_save_keeps_last_real_save_metric
No-op under nextest, which isolates each test in its own process.
