fix: stop working-set metadata retry storm

[OpenCow42]

Summary

• model the File Provider working-set identifier explicitly and preserve raw-value round trips
• return noSuchItem for working-set metadata lookups before runtime loading, networking, or error auditing
• reject the working set as a mutation parent and document the virtual-container lifecycle
• extend identifier regression coverage while preserving root, trash, numeric, and invalid-value behavior

Validation

• focused identifier regression test passed on macOS, iOS Simulator, and visionOS Simulator
• normal macOS build passed
• generic visionOS build passed
• SQLite quick_check returned ok; no schema or historical audit rows were changed
• git diff --check passed

Existing test-suite issue

The complete macOS and iOS scheme runs do not terminate because PotassiumProviderCoreTests/kdriveServiceCreatesFileWithVersionConflictStrategy() remains suspended at operation.value. Before cancellation, macOS reported 117 passing tests and iOS reported 104 passing tests, including this change’s regression test. The iOS launch-performance test was also canceled when the stuck run was stopped.