chore: prepare 0.1.0 release for crates.io

[coderdan]

Summary

Pre-publication tidy-up before pushing 0.1.0 to crates.io.

• Update Cargo.toml description so it reflects both NEON and AVX2 (the original wording was written when only NEON encode existed); add homepage; add [package.metadata.docs.rs] so docs.rs renders both NeonEncoder and Avx2Encoder.
• Mark `DecodeError` as `#[non_exhaustive]` so future variants aren't breaking. First publish is the cheap moment to do this.
• Add `CHANGELOG.md` with an initial 0.1.0 entry (Keep-a-Changelog format).

Verification

• `cargo fmt --check` clean
• `cargo clippy --all-targets -- -D warnings` clean on stable + 1.85, on aarch64-apple-darwin and x86_64-apple-darwin
• `cargo test` — 24 lib + 18 parity + 1 doctest, all green
• `cargo package --list` — 18 files, xlsx/mise.toml/DS_Store correctly excluded
• `cargo publish --dry-run` — 38 KiB compressed, no warnings
• `RUSTDOCFLAGS="-D warnings --cfg docsrs" cargo doc --no-deps` clean

Test plan

• CI green
• Eyeball the rendered Cargo.toml description on crates.io after publish (post-merge)