test: 8 shutdown tests — EXIT, AtExit, WorkArea, panic-safe, statics

Tests verify:
- EXIT PROCEDURE auto-execution on shutdown
- Reverse module order for EXIT
- AtExit LIFO callback order
- WorkArea CloseAll on exit
- Panic in cleanup doesn't crash (safeCall)
- Shutdown runs exactly once (sync.Once)
- Static variables cleared
- Full sequence order: EXIT → AtExit → WA:Close → onExit

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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