Download SerDe snapshots from datasketches-tck

[tisonkun]

Summary

• replace the C++ checkout, build, and snapshot generation steps with downloads from a pinned apache/datasketches-tck revision
• run the existing C++ and Go SerDe regression profiles as separate workflow matrix jobs
• add a reusable Bash downloader that extracts only the requested language snapshots and replaces stale local test data
• rename the workflow file from cpp-serde-compat.yml to serde-compat.yml now that it covers both languages

The pinned TCK revision matches the one currently used by datasketches-rust. datasketches-java does have a Go regression profile; it currently covers the CPC cross-language case, so this PR enables it in CI with TCK snapshots as well.

Testing

• tools/download_serialization_test_data.sh (223 C++ snapshots and 251 Go snapshots)
• mvn test -P check-cpp-files (38 tests)
• mvn test -P check-go-files (1 test)
• mvn test (2,279 tests)
• mvn verify -DskipTests
• Bash syntax, workflow YAML, and git diff checks