Theta: fix EmptyCompactSketch preamble check for historical images

[leerho]

Summary

EmptyCompactSketch decides whether the first 8 bytes of a serial version 3 compact image are a valid empty sketch, using a mask (which bits are examined) and a test (their required values). This PR corrects both so that every empty image written by an open-source Java or C++ release is accepted, while mangled images are still rejected. The matching C++ change is apache/datasketches-cpp#543, which uses the same mask and test values.

	9.0.0	This PR
Mask	0X00_00_EB_00_00_FF_FF_FFL	0X00_00_EF_FF_FF_FF_FF_FFL
Test	0X00_00_0A_00_00_03_03_01L	0X00_00_0E_00_00_03_03_01L
Legacy test (preamble longs 3)	none	0X00_00_0E_00_00_03_03_03L
Array written	{1, 3, 3, 0, 0, 0x1E, 0, 0}	unchanged

The mask and test now satisfy two rules, checked by a unit test: the test lies within the mask, and our own array passes.

Changes

• Ordered flag ignored. C++ before 3.3.0 and Java before 1.0.0 wrote empty images without it (0x0E). The current mask on main (from a9954b9, not in any release) requires it.
• Bytes 3 and 4 must be zero. They have been "not used by compact" since 0.1.0. 9.0.0 ignored them.
• Reserved flag bits 0, 6 and 7 must be clear. Any future use of them requires a new serial version.
• Empty flag checked. Both callers had already confirmed it, so this only makes the test complete.
• Legacy 24-byte form accepted. Before 1.0.0, Java wrote empty sketches with p < 1 with preamble longs 3 and theta = p. wrap has rejected these since 2019. They are now accepted if they have no retained entries and carry the expected seed hash, and are read as EmptyCompactSketch with theta = 1.0.
• CompactThetaSketch.heapify now applies the same check as wrap. Before, it accepted any image with the empty flag set.

No serial version change: the writer is unchanged. Serial version 4 is not affected, because empty sketches are never written compressed.

Testing

• Full suite passes (2290 tests).
• New EmptyTest cases check the two rules, accept 5 historical images and reject 12 mangled ones, through heapify, wrap(MemorySegment) and wrap(byte[]).
• Real images read as empty by heapify and both wrap overloads: 2 from sketches-core 0.9.0 (p = 0.5, ordered and unordered), and 60 written by C++ 1.0.0-incubating, 2.0.0-incubating, 3.0.0, 3.2.0, 3.3.0 and current master (10 empty cases each: update compact, union, intersection and A-not-B, with p = 1 and 0.5, ordered and unordered).

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