A seeded fork (a dsh-better-sidebar Side tab, a subagent_fork child,
a UI fork) replays its parent's event log verbatim, usage settlements
included, so its cumulative cost fold re-priced history the parent
session had already booked - a family of idle Side tabs billed the
parent's whole history again (#94).
The fold now zeroes the session-cost totals at the tagged
`session/end-seed` marker the session core stamps at the inherited
cut, in both projection units: the timeline's cost book and the
activity day ledger (whose armed step slot dies with the cut, so a
child settlement cannot attribute to a parent day). Untagged resume
markers reset nothing; nested seeds re-mark at their own cut, so a
session's totals are always its own post-seed spend. stateVersion
bumps (timeline 22, activity 3) refold cached rows from the log on
upgrade.