v0.5.1 — crates.io description + keywords fix

Metadata-only patch. No API change, no on-disk format change, no migration.

v0.5.0 shipped the reframing ("high-throughput local buffer for cloud sync") in the README and AGENTS.md, but the three highest-visibility discovery surfaces still carried the old "durable bounded queue" framing. This release fixes all three.

What changed

• Cargo.toml description (→ crates.io search results): now leads with "High-throughput local buffer for cloud sync" (was "Durable bounded queue"). This is the #1 discovery path for new users.
• Cargo.toml keywords: ["queue", "cloud-sync", "spool", "segment", "buffer"] (was ["queue", "disk", "durable", "segment", "buffer"]). Dropped the generic "disk"/"durable" in favor of target-audience terms.
• Crate-root rustdoc (src/lib.rs //!): docs.rs module page and IDE hovers now show the cloud-sync positioning.
• SegmentBuffer<T> struct doc: same reframing.

Also fixed: the CHANGELOG [0.5.0] section header was missing (a multiedit silently failed during the v0.5.0 release session, leaving the v0.5.0 content under [Unreleased]). The header structure is now correct.

Upgrade

cargo update -p segment-buffer

Drop-in from v0.5.0 — no code changes required.

Verification

All gates green on the tagged commit (8994421):

• cargo fmt, cargo clippy -- -D warnings, cargo test --features encryption (96 + 38 passed)
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --features encryption
• cargo publish --dry-run
• CI (all 10 jobs) + Nix workflows: success