ZIP DIF handling, size budgets, and oversized-entry semantics

prepareZipDifs returning any non-null value, including [], means the path was handled as a ZIP; null means fall through to normal file handling because it was not a ZIP or the archive was skipped wholesale. maxZipTotalSize is a per-archive cumulative uncompressed/container budget, distinct from per-entry maxFileSize; its default is 2 GiB and 0 disables it. Pass it to readZipDifEntries as maxTotalSize. Oversized compressed entries only warn inside readZipDifEntries: their format is unknown before inflation, so they must not increment prepareDifs’ exit-driving oversizedCount. That counter is reserved for format-accurate on-disk files.