server: fix stale eviction policy comments + clarify ring buffer log format

The create_checkpoint() comment described the old "evict highest pos_min"
policy that was replaced by insertion-order eviction in dbedc90ec. The
comment argued against insertion-order eviction, contradicting the actual
code below it. Also fixed a stale TODO about pos_min/pos_max ranges that
is already handled by the LCP acceptance predicate's n_swa guard.

Log format: "slot N/M" was ambiguous (looked like "slot number 16" when
it means "16 of 16 checkpoints"). Changed to "(N/M full, ...)" to make
it clear this is a count, not a slot identifier. This addresses user
confusion seeing "recycled slot 16/16" and thinking slot 16 was the
newest being recycled instead of the oldest.