…deps

After GenerateRules has run for every package, rewrite two marker-delimited
sections at the repo root:

  - Cargo.toml between `# gazelle:proto_rust_members start/end` —
    populated with one entry per package that emitted a proto_rust_library.
  - BUILD.bazel between `# gazelle:vendor_proto_sources_deps start/end` —
    populated with the underlying _lib target of each proto_rust_library
    plus every proto_compiled_sources rule.

Both rewrites are no-ops when the markers (or the target file) are absent,
so existing repos without the markers see no change. Entries are sorted
and deduplicated; the file is only rewritten when content actually changes.