Merge order, duplicate files & resource release on cancel
Merge reads a documents array with at least 2 documents. Put a 2-page A first and a 3-page B second: plan.pageIds is ["A1","A2","B1","B2","B3"] (5 pages), and thumbnailOrder exactly matches input order. Repeating the same document is never silently de-duplicated: A+A gives 4 pages ["A1","A2","A1","A2"] and duplicateSourceIds:["A"].
The default budget is 50 MiB / 200 pages in total; exceeding it fails with BUDGET_EXCEEDED before planning. Encrypted, corrupt and empty inputs return PASSWORD_NOT_SUPPORTED, CORRUPT_FILE and EMPTY_FILE; passwords are currently unsupported.
Lifecycle semantics are defined by the pure lifecycleReducer: after three consecutive import → export → release cycles, all 6 registered Blob URLs and 3 worker handles are in the released lists with zero live handles. Cancelling enters the cancelled state, releases handles immediately and blocks any later automatic or explicit download (blockedAutoDownloads increments while downloads stays 0). No real worker or Blob URL is started this round; browser behaviour is not tested.