fix: align empty Theta and Tuple seed hash handling

[tisonkun]

Empty compact Theta and Tuple sketches currently serialize their configured seed hash, so the empty image depends on its seed. Reading an empty image also exposes the stored fingerprint instead of the caller's, and legacy Theta v2 rejects empty images with zero or mismatched seed hashes.

Use canonical 8-byte images with a zero seed hash for empty serialization, including Theta's compressed fallback. Empty deserialization associates the result with the supplied seed; for example, reading the same empty bytes with seed 123 or 456 gives the corresponding in-memory fingerprint. Non-empty sketches still require a matching seed, including sampled sketches with no retained entries.

This aligns the wire encoding with apache/datasketches-java#772 and empty-v2 acceptance with Java's legacy reader. Rust objects retain the caller's seed fingerprint rather than Java's zero-valued empty-object fingerprint.

Validation:

• Compare custom-seed empty output against the pinned C++ snapshots and check caller-seed association for zero and mismatched stored hashes.
• Port the three empty-v2 layouts from Java's ForwardCompatibilityTest.
• Verify that sampled, non-empty sketches with no retained entries preserve theta and still reject seed mismatches.
• cargo x check, cargo x test, and cargo x lint.