The contextActivity view copied each day entry shallowly while its own
comment promised the registry, the cache writer and the wire could never
mutate the fold's record. Since stateVersion 2 and 4 the entry carries a
four-level `cost` ledger and a `skills` table, and both stayed aliased to
the persisted state — the exact rule foldWire.ts states for its own served
fields and honours by copying every level of `cost`.
The copy is conditional-spread throughout: one `undefined`-valued property
fails the whole lossless-JSON push. The ledger walk moves beside
addBilledUsage as its mirror, so the timeline and activity halves price and
serve through one implementation.
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