[kvlonge]

Why

Native shutdown polling should not allocate a Python threading.Event or call Python just to read and set an internal exit flag.

Changes

• Replace the private process-wide event with a monotonic native atomic flag.
• Preserve existing exit hooks, worker notifications, joins, bounded waits, interruption behavior and public interfaces.
• Remove the helper's four Python-crossing allowances. Document a symbol-scoped static-state exception for the same existing interpreter-wide shutdown signal; no new lifecycle state or Python-boundary exception.

Verification

• Rebuilt the extension with make fast-native; its exit signal is a one-byte native symbol.
• 76 focused tests passed, including subprocess finalization, timed-out sinks, unjoined planning and real DuckDB CLI planning.
• cargo test -p sqlbuild-python --profile fast --lib: 9 passed.
• Installed all extras and passed exact make check-ci, including workspace clippy, types, strict-adapter checks and zero fensu faults.
• One bounded independent read-only review found no blockers. Duplicate analysis and private hygiene scans passed.
• No DuckShop addition: internal synchronization-primitive refactor; no new project shape, bug fix or lifecycle behavior. Existing finalization scenarios cover the preserved contract. No performance claim or budget increase.