test: expand Windows integration smoke

[IvanLarinDev]

Summary

• add a real isolated Explorer SendTo install/uninstall round-trip that validates the generated .lnk
• add a real FileSystemWatcher burst-routing and locked-file cancellation smoke test
• keep production SendTo wrappers behavior-compatible while exposing only internal path-scoped test seams
• isolate watcher config/error logging and drain in-flight work before restoring global test state

TDD evidence

• RED: isolated SendTo overloads did not exist
• GREEN: both new Windows integration scenarios passed
• independent exact-snapshot review initially blocked unsafe error-log isolation; a second RED/GREEN cycle fixed it
• final reviewed tree: 54cfe978f519df0e44dad74a6087ba636a427360
• final raw diff: cad0390432d66498d1c38a5cf1754be5164e2d1d (14,032 bytes)

Verification

• locked restore
• Release build with warnings as errors: 0 warnings, 0 errors
• 523 unit tests
• 11 Windows integration tests
• framework-dependent and self-contained lifecycle smoke
• framework-dependent and self-contained Release publish
• scoped formatter and cached diff checks
• independent fail-closed exact-snapshot review: Ready