build(rust): track Cargo.lock (binary crate)

Per .gitignore line 27 comment 'Keep for binaries' — this is a
[[bin]] crate (main.rs), so Cargo.lock pins dependency versions for
reproducible builds.

Not pushed: this commit sits alongside 5 other local commits on
claude/upbeat-mendel-lBO9G, all held back pending PR #42 merging
into main so the rebase + estate-policy adaptation can land in
one clean sweep.

Author: claude