Keep transaction ids monotonic across process restarts

[maxweber]

Summary

• compare each generated transaction SQUUID with the database's current basis
• when the process-local generator is behind, increment the persisted basis instead
• add a regression test proving a later update remains visible when the generator goes backwards

Why

colossal-squuid keeps its monotonic state in memory. After a process restart, or when the clock moves backwards, it can generate a transaction id below the latest id already stored by dbval.

Because dbval uses transaction-id ordering to determine the current datom, such a transaction could commit successfully while its update remained invisible. The database basis also stayed at the older, higher transaction id.

The persisted database basis is the source of truth, so every new transaction id must be strictly greater than it.

Verification

• script/test_clj.sh — 172 tests, 1079 assertions, 0 failures
• cd store-slatedb && clojure -M:test — 2 tests, 11 assertions, 0 failures