Make test backend ownership and conditional writes atomic

[jeregrine]

Problem

The consistency-probe backend checks ETS before inserting or deleting. Concurrent claims can both succeed, and conditional writes/deletes can consume the same ETag more than once. This gives concurrency regressions a storage model that violates the backend contract.

Fix

Use atomic ETS insert-new, select-replace, select-delete, and take operations. Keep fault overrides and call recording intact. Treat stored values as literal match-spec data.

Add focused contract cases for contended creation, writes sharing a token, write/delete races, and stale tokens after deletion and replacement.

Supporting information

This changes only the test backend. Production adapters and their consistency semantics are unchanged.