chore: prepare 0.2.1 release

[houseme]

What

Prepare the rustfs-uring 0.2.1 patch release. There is exactly one unreleased commit since the 0.2.0 tag — #13 — and this cuts it into a release.

Changes since 0.2.0

• Changed — the driver's runtime diagnostics (submit-error, CQ-overflow, bounded-drain, shutdown-timeout) now route through tracing with structured fields instead of eprintln!, unifying with the RustFS tracing pipeline for filterable observability. The panic-abort barrier keeps its direct stderr write, since a subscriber may not flush before abort().
• Docs — the README was rewritten from the current public API and trimmed; docs/DESIGN.md was removed for a single source of truth.

No public API changed and the read path / cancel-safety ownership model are untouched, so this is a patch bump.

This PR

• Bumps the crate version 0.2.0 → 0.2.1 (Cargo.toml, and the workspace Cargo.lock self-entry).
• Promotes the CHANGELOG [Unreleased] section to [0.2.1] - 2026-07-11, attaches the #13 reference to both entries, keeps an empty [Unreleased], and adds the 0.2.1 compare links.
• Updates the README install snippet to 0.2.1.

Verification

• cargo build (non-Linux stub) and cargo check --target x86_64-unknown-linux-gnu (compiles the Linux-only driver module) both pass.
• cargo clippy --target x86_64-unknown-linux-gnu --all-targets -- -D warnings is clean.
• Crate version, README snippet, and lockfile self-entry all read 0.2.1.

Publish (not done here)

This PR only stages the release. Tagging 0.2.1 and cargo publish are left to a maintainer after merge. Once 0.2.1 is on crates.io, the companion bump in rustfs/rustfs (#4730, currently pinning 0.2.0) can be moved to 0.2.1.