apache / datasketches-cpp / 35230358712

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Write the single-item flag on a compact theta sketch (#526)

Java sets bit 5 of the flags byte (SINGLEITEM_FLAG_MASK) on a compact
sketch holding exactly one entry in exact mode. C++ never wrote it, so
the one-item image was the last theta fixture that differed between the
two languages: byte 5 was 0x1A here and 0x3A in Java.

The condition mirrors Java's: one retained entry and theta at its
maximum. Only the two version-3 writers change; the compressed form
never carries a single item. Readers are untouched, since they test
individual bits and have always ignored this one, which is also why C++
already reads Java's single-item images.

Pins the exact 8-byte empty and 16-byte single-item images in tests,
with a non-default seed to show the seed hash is absent from one and
present in the other, and checks that one entry in estimation mode does
not get the flag.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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