Prepare crates.io publishing

[adsharma]

Summary

• add crates.io metadata and restrict packaged files to the Rust binding sources
• download Icebug into OUT_DIR when building from a packaged crate so Cargo verification does not mutate the source archive
• allow the download script to target an explicit vendor directory

Validation

• cargo fmt -- --check
• cargo check --locked
• cargo test --locked
• cargo package --allow-dirty
• cargo publish --dry-run --allow-dirty