Mirrors the read path added in 47277c7: updateSaveDataWithStruct now
flushes Zenny/GZenny/CP back to the blob for ZZ, using the same
`ok && off > 0 && off+size <= len(blob)` guard so unmapped modes remain
inert.
Tests lock down byte-level idempotence — the most important invariant
for save data. Parsing a live kirito ZZ blob and immediately writing
the struct back produces a byte-identical blob, so enabling these
fields cannot silently corrupt existing player saves on the next save
cycle. Additional coverage: round-trip through both paths, non-ZZ
modes never touch the blob bytes, and truncated blobs don't panic on
write.
b1972e3c96