Theta preamble diagnostic misreports historical single-item images #773

Problem

theta.PreambleUtil.preambleToString() derives CurrentCount for one-preamble-long images from SINGLEITEM_FLAG_MASK.

Historical C++ single-item images may omit that flag. Java accepts those images because the theta parser deliberately does not depend on the flag, but the diagnostic output reports CurrentCount: 0 even though the image contains one retained item.

This was identified while resolving the cross-language differences tracked in apache/datasketches-cpp#460. Current C++ now writes the flag, but older valid images still expose the diagnostic mismatch.

Expected behavior

For a one-preamble-long theta image, derive the diagnostic count from the empty flag:

• empty image: CurrentCount: 0
• non-empty image: CurrentCount: 1

The printed SINGLE_ITEM field should continue to reflect the serialized flag itself.

Proposed test

Serialize a single-item compact sketch, clear only the single-item flag to model a historical C++ image, and verify that preambleToString() reports CurrentCount: 1 while preserving SINGLE_ITEM: false.