fix(token-usage): drop the superseded bucket index; spec the clamp carve-out

The v4 expression index fully supersedes v3's plain bucket index (no
remaining query uses it — verified via EXPLAIN QUERY PLAN), so v4 now
drops it instead of maintaining a dead B-tree on every entry write.
The spec's repricing contract also carves out entries clamped at the
$10k per-entry ceiling: their token counts are not clamped, so such
buckets reprice approximately, like transcript-priced ones.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>