checkpoint: align contract terminology to persistent

Address Copilot review on #1504: the verbatim-moved WriteOptions/UpdateOptions doc comments still said 'committed checkpoint' (the store concept) — reword to 'persistent checkpoint' to match the rest of the api/checkpoint contract. Rename the ReadCheckpoint tests (TestReadCommittedCheckpoint* -> TestReadCheckpoint*) to match the renamed function.

References to the git 'committed tree' and the TotalCommitted/total_committed attribution field are left as-is — those correctly describe git commits, not the store.