fold.ts held the persisted state shape, the retention bounds, the untrusted-usage
sanitizer with its accumulators, the surface projection, and the served-value
builders at 1234 lines. The state, the metering accumulators, and the wire builders
move into flat siblings; applyTimeline and the surface projection stay together
because the clone contract they enforce spans both.
The metering extraction also unifies BilledUsage with the shared CostBucketTotals:
one four-bucket vocabulary, so the accumulator input is spelled `uncached` like the
totals it feeds. The persisted and wire shapes are unchanged (both already carried
`uncached`), so no stateVersion bump and no cached-row invalidation.
Production callers import the owning sibling; fold.ts keeps re-exporting the
pre-split surface for the specs.
bowenliang123
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