refactor(req): remove ReqUnion

[tisonkun]

Summary

• remove ReqUnion, which only delegated to an inner ReqSketch
• keep ReqSketch::merge as the REQ aggregation API and add a usage example
• update the README and unreleased changelog to describe merging without a separate union type

Rationale

The Java, C++, Python, and Go REQ implementations expose aggregation directly through the sketch's merge method. Unlike CPC, HLL, and Theta unions, ReqUnion maintained no distinct accumulator state or configuration-reconciliation behavior, and to_sketch cloned the wrapped sketch.

REQ has not been included in a release yet, so the discarded intermediate API does not require a migration entry. This change intentionally leaves the existing ReqSketch::merge compatibility rules, including mixed-k behavior, unchanged.

Testing

• cargo x check
• cargo x test
• cargo x lint

Closes #217.

[tisonkun]

@pmcgleenon Seems ReqUnion is almost identical to continuous calls to ReqSketch::merge. I tend to reduce concepts from users and add them only if needed or if concrete use cases occur.

[tisonkun]

... to be clear, a merge is not a final decision. You can always do review after commits and revert a commit with a reason.