nzb-nntp v0.2.20

Changes

• document that Downloader is the simple sequential failover path, not the high-throughput pooled pipeline path
• clarify crate and README guidance for consumers that need persistent pooled connections and pipelining
• add regression coverage proving Downloader opens a fresh connection per article attempt

Validation

• cargo fmt
• cargo clippy --all-targets -- -D warnings
• cargo test -- --nocapture