The review's arithmetic checks: the closing-frame walk used cap
logBudget - openCost, so a compliant merged entry (58-byte wire cost under a
64-byte budget) could see a negative cap and truncate; the first-fragment cap
used logBudget instead of the ledger's logBudget - 1, so an open frame costing
63 was admitted with a bill of 64, pushing the ledger negative and letting a
subsequent empty frame ride in one byte past the configured cap; and the child
billed a closing frame as a fresh entry (quotes+separator again) instead of the
merged tail, truncating an exact-fit 30+30 entry.
Fixes: first-fragment cap logBudget - 1 (matching admit), continuation and
closing-frame cap logBudget + 2 (billed without quotes), jsonStringCostUpTo
returns undefined below 2 bytes, and the child's split billing keys off
_open_started alone (a closing frame pays content only) with the cheaper bound
len(text) while a merge is open. Regression cases cover all three arithmetic
paths.
3001cc23be