feat(theta): add compact sketch serialization

[jonbjo]

Summary

Add binary serialization support for Theta sketches, compatible with Apache DataSketches Java and C++ implementations.

Changes

• Add CompactThetaSketch type with serialize() and deserialize() methods
• Add convenience methods on ThetaSketch: compact(), serialize(), deserialize()
• Support all compact sketch formats: empty, single-item, exact mode, and estimation mode
• Handle legacy seed_hash=0 format for backward compatibility
• Add cross-language compatibility tests using Java-generated test data

Motivation

This enables reading and writing Theta sketches in formats used by Iceberg Puffin files.

Limitations

This PR focuses on the standard compact format. Identified features not included:

• Compressed format - Java's toByteArrayCompressed() uses bit-packing for smaller size; not supported
• Non-compact format - Only compact sketches can be deserialized
• Single-item serialization optimization - We deserialize single-item format but always serialize using the standard exact-mode format (functionally correct, just slightly larger for single-item sketches)

These could be added in follow-up PRs if needed.

Testing

• Unit tests for serialization round-trips
• Cross-language compatibility tests that deserialize sketches generated by datasketches-java

[tisonkun]

This can be duplicated with #77.

As #77 comes first, I'd review that patch first.

@jonbjo you may take a look at #77 and see if we can collaborate there. Besides, I'm interested how you spot this issue since I see two PRs now duplicate with #77 and I'm curious about how it happens.

[jonbjo]

You are right, #77 covers this PR and more. My bad to not check existing PRs before pushing. I needed serialisation for Apache Polaris rust library since it is used for table stats in Puffin files and I implemented it earlier. As I was cleaning up the code I figured I would push this part upstream.