A new era must be implemented to turn on RLE

In the randomized leader election scheme, the block seal format changed. Therefore, a new era must be introduced to distinguish old and new blocks

The issue names randomized leader election, block seal formats, and eras, but no files, tests, or entry points. Start by locating the RLE implementation and block seal handling, then trace how blocks are distinguished across eras. Done should mean old and new seal formats are distinguishable and RLE can be enabled, with coverage for both formats.

Written by the indexing model from the issue text.