Theta: validate the preamble of empty compact sketches as Java does

[leerho]

Summary

C++ accepted any serial version 3 compact image with the empty flag set, without checking the rest of the preamble. This PR applies the same check as Java EmptyCompactSketch (apache/datasketches-java#776), using the same mask and test values, so both languages accept and reject the same empty images.

	Value
Mask	0x0000EFFFFFFFFFFF
Test (preamble longs 1)	0x00000E0000030301
Legacy test (preamble longs 3)	0x00000E0000030303

Changes

• New compact_theta_sketch_parser::check_empty_v3, used by parse (byte deserialize and wrap) and by stream deserialize_v3.
• Checked: preamble longs, serial version, sketch type, bytes 3 and 4 zero, read-only, empty and compact set, single-item and reserved bits 0, 6 and 7 clear.
• Ordered flag ignored. C++ before 3.3.0 and Java before 1.0.0 wrote empty images without it.
• Seed hash ignored in the 8-byte form. Writers have used both 0 and the real seed hash.
• Legacy 24-byte form accepted. Before 1.0.0, Java wrote empty sketches with p < 1 with preamble longs 3. These must have no entries and carry the expected seed hash. C++ never wrote this form.
• Stream position fixed. For a legacy image, stream deserialize_v3 now reads all 24 bytes instead of stopping after 8.

Behavior change for users: empty images with non-zero bytes 3 and 4, read-only or compact missing, single-item or a reserved bit set, preamble longs 2, or a malformed legacy form are now rejected. No C++ or Java release I tested writes any of these.

Testing

• Full suite passes (17/17).
• Three new test cases mirror the Java ones: the mask and test rules, 5 historical images accepted and 12 mangled images rejected, each through stream deserialize, byte deserialize and wrap.
• Real images read as empty through all three entry points: 60 written by C++ 1.0.0-incubating, 2.0.0-incubating, 3.0.0, 3.2.0, 3.3.0 and current master, and 2 from Java sketches-core 0.9.0.

🤖 Generated with Claude Code

[coveralls]

Coverage Report for CI Build 37711570993

Coverage increased (+0.03%) to 82.409%

Details

• Coverage increased (+0.03%) from the base build.
• Patch coverage: 26 of 26 lines across 2 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	21454
Covered Lines:	17680
Line Coverage:	82.41%
Coverage Strength:	1371453.49 hits per line

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💛 - Coveralls