Move ref normalization into Rust

[t-kalinowski]

Summary

• Move package ref normalization out of the R resolver and into Rust so the R session stays thinner.
• Keep the resolver focused on pak/renv work while Rust now normalizes pkg>=... and pkg==... refs before sending them on stdin.
• Preserve the public CLI behavior with a regression test that checks normalized refs share the same resolution cache entry.

Testing

• cargo test
• cargo check