- Card creation now owns mined-card counting; remove the outer single and
multi-line mining increments that double counted each card.
- Preserve fractional first_seen/last_seen precision when session deletes
compare and refresh lexical aggregates.
- Derive vocabulary_visible from the stored word row on every write so the
new-word charts agree with the filtered vocabulary summary.
- Bump the lexical rollup version so existing databases re-derive visibility
and repair stale last_seen values once in the background worker.
