[AGiorgetti]

Summary

Closes #107.

• Move the checkpoint read inside the existing synchronous and asynchronous cleanup guards.
• Reject disposed engines before collection access, translate MongoDB read failures exactly once, and never delete after a failed read.
• Preserve cancellation/token identity, genuinely async acquisition and deletion, cursor disposal, query shape and the strict checkpoint cutoff.
• Document the exception compatibility change and the expanded callback scope for protected virtual guard overrides. No API/schema migration or MongoDB minimum-version change.

Verification

• Test-first on unchanged production code: 32 failed, 56 passed, 0 skipped across 88 new assertions.
• After the fix: 598 passed, 0 skipped on each of net8.0, net9.0 and net10.0, covering [P2] Guard recycle-bin checkpoint reads before I/O in sync and async cleanup #107, [P2] Async checkpoint reads execute FindSync before returning their Task #88 and synchronous enumeration guards.
• Full Release solution build: 0 errors across all configured targets. Existing compiler/analyzer and NuGet vulnerability warnings remain; dependencies were not changed.
• Independent review found no blockers.
• CI run 42237139928 passed for d7bd8cb06d3223627b44c5aac86246f39f66335e: Windows and Ubuntu builds succeeded; the full MongoDB-backed suite passed 1,138 tests, 0 failed, 0 skipped on each of net8.0, net9.0 and net10.0. This includes cleanup with the highest checkpoint inside/outside the recycle bin.
• Local MongoDB startup was unavailable in the restricted cloud shell; the successful CI MongoDB 9.0.4 service jobs provide the real-database verification.

Co-authored by Alessandro Giorgetti and Codex (OpenAI).