feat: crate metadata and merge operation

[MovAh13h]

Two additions in one branch — future work will be split into individual PRs.

• Added crate metadata to Cargo.toml: description, license, repository, keywords, categories
• Added LICENSE (MIT)
• Added BloomFilter::merge(&self, other: &Self) — creates a new filter containing the union of both filters
• Added AtomicBloomFilter::merge(&self, other: &Self) — snapshot union into a new filter
• Added AtomicBloomFilter::merge_from(&self, other: &Self) — atomic in-place OR using fetch_or(Release)
• Added BloomError::IncompatibleGeometry — returned when filters have different m or k
• Added tests covering union correctness, identity merge, item count, and incompatible geometry error