Round vs. epoch

`round` and `epoch` are used interchangeably in many places across the spec. Although they're official synonyms, this causes confusion for implementers who have not otherwise stumbled across that fact.

We should do a pass and replace all mentions of round with epoch where reasonable, and especially in code or pseudocode. For example, `ROUND_CUTOFF` should be `EPOCH_CUTOFF`.

**Priority:** Not urgent, can be done after spec is feature complete.

cc @ZenGround0 @sternhenri

Search the specification for uses of “round,” prioritizing code and pseudocode, and inspect the ROUND_CUTOFF example in context. Review each occurrence for whether “epoch” is an appropriate replacement, then confirm that terminology is consistent throughout the specification without changing cases where “round” remains clearer.

Written by the indexing model from the issue text.