Enforce unsafe practices with clippy

There's two clippy lints we could enable which would make us better about handling unsafe code:

• missing_safety_doc
• undocumented_unsafe_blocks

The rational and examples of these can be found at https://std-dev-guide.rust-lang.org/policy/safety-comments.html

We can do this on a crate-by-crate basis to keep each PR small. I think lance-jni can be out of scope, as it's low value. Here's the modules that could benefit most:

• lance-linag, all the SIMD bindings
• lance-index, particularly the PQ distance tables, RaBitQ unpacking
• lance-encoding
• lance-io, particularly io-uring buffer-pointer sites

Choose one listed crate, such as lance-linag, lance-index, lance-encoding, or lance-io, and review its SIMD, PQ, RaBitQ, or io-uring unsafe sites. Read the linked Clippy lint and Rust safety-comments guidance first, then run Clippy for the selected crate. Done means the two lints are enabled and the selected unsafe code has the required safety documentation or comments.