checkpoint: rename store impl/test files to persistent/ephemeral

Pure file renames (no content change) to match the vocabulary: committed.go -> persistent.go committed_write.go -> persistent_write.go committed_reader_resolve.go -> persistent_reader.go temporary.go -> ephemeral.go committed_test.go / temporary_test.go -> persistent/ephemeral_test.go

committed_refs.go is left as-is pending the CommittedRefs type rename.