refactor: keep sketch configuration constants private

[tisonkun]

Summary

• keep the REQ DEFAULT_K, MIN_K, and MAX_K values internal
• keep the Bloom filter builder's size and hash-count bounds internal while preserving validation behavior
• document the released Bloom filter API migration in the Unreleased changelog

Rationale

Sketch defaults and validity constraints are already expressed through Default, builders, and fallible construction. The REQ range constants also did not express the complete even-k requirement, while the Bloom filter maximum bit count reflects a serialization-field ceiling rather than a portable allocation guarantee.

Testing

• cargo x check
• cargo x test
• cargo x lint
• verified that the generated rustdoc item list and search index no longer expose the seven constants