Test: add Bundler (Gemfile.lock) edge-case fixture for vg build

[vibgrate-team]

Context

We already seed edge-case fixtures for pnpm, Yarn Berry, Cargo workspaces, Go go.work, and Poetry. Bundler (Gemfile / Gemfile.lock) is a common Ruby path that still needs a small deterministic fixture so vg build graph extraction stays stable across platforms.

Newcomer-scoped: add fixture + assertions only; no new ecosystem resolver design.

What to do

1. Add a minimal Bundler fixture under test/ (or existing fixture layout) covering at least one edge: path gem, git gem, or platform-specific gem entry.
2. Assert deterministic graph.json (stable IDs / sorts; no filesystem-order dependence).
3. Document the fixture purpose in a one-line comment or fixture README stub if that is the repo convention.

Acceptance

• Fixture checked in; pnpm test covers it
• Same input → same graph across runs
• DCO sign-off; prefer vg in any doc mention