Move the accumulated raw BM25 file-backed work into a 0.2.4 changelog section and bump the crate version for publish. The release includes the postings 0.2.6 dependency bump and uses postings' raw doc_id_range API for dense-range planning.

Verification: cargo fmt --check; cargo test --all-features; cargo clippy --all-targets --all-features -- -D warnings; cargo semver-checks; cargo run --example raw_bm25_segment --features raw-segment.