Always assign custom master indices (>= FIRSTMASTER=2) to all files,
never SuperMaster (index 0). The original's ExtractFiles() routes
SuperMaster files through a code path that hangs. The original itself
never uses SuperMaster in file COMPRESS records — it always creates
at least one custom master, even for archives without dedup groups.
For ungrouped files, a default custom master is built from the largest
file's first 64KB. All files reference this master, matching the
original's archive structure.
The automated DOSBox-X test now validates multi-file round-trip in
both directions: 4 files UC2 v3 -> original, 5 files original -> UC2 v3.
All content verified byte-for-byte.
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