add deterministic VSS durability gap tests - #142

Cherry-picked the deterministic durability test commit into #138 as fdc814c (branch hardik/rln-kv-vss-durability). All three tests pass locally against the durability implementation, and the full PR 138 matrix is running. PR 142 remains useful as the independent red-on-dev / green-with-138 evidence.

Final follow-up: the three deterministic tests from this PR are in #138, and #138 now also includes a fourth deterministic shutdown-admission regression found during final review. That test proves a retry drain cannot begin remote VSS I/O after stop() returns. The final #138 head passed all 49 required checks, including VSS E2E/restore/re-import, process-kill matrices, AddressSanitizer, ThreadSanitizer, SDK E2E, and platform artifacts: https://github.com/UTEXO-Protocol/rgb-lightning-node/pull/138/checks

Closing this now in favor of #138