Add real-world lockfile fixtures for Go/Rust/Gradle parsers (#24)

Matches the existing testdata/lockfiles/*-real/ pattern already used for
Python (uv-real, poetry-real, pdm-real, requirements-txt-real). The new
fixtures are vendored from popular open-source projects and exercised
via the auto-discovered TestSnapshots harness on every CI run:

- gomod-real: gohugoio/hugo go.mod + go.sum (184 components)
- cargo-real: clap-rs/clap Cargo.lock (455 components)
- gradle-real: Netflix/maestro maestro-aws gradle.lockfile (83 components)

These snapshots catch parser regressions against real-world lockfile
shapes such as long replace directive chains, /v2+ module paths,
workspace members, git/registry source mixes, multi-version crates,
and large Gradle configuration CSVs.

Refs #22

Author: sns45