Test: add Yarn Berry (yarn.lock v2+) edge-case fixture for vg build

[vibgrate-team]

Context

Yarn Berry lockfiles differ from classic Yarn/npm. A pinned fixture keeps vg build dependency edges deterministic for Yarn Berry trees (CONTRIBUTING: same input → same graph.json). Complements the pnpm catalog fixture (#211) without overlapping it.

What to do

1. Add a minimal Yarn Berry project fixture (checked-in yarn.lock, no network in tests).
2. Assert stable graph edges for direct deps (and note workspace protocol if used).
3. Keep the fixture small enough for CI.

Acceptance

• Fixture checked in; tests run offline
• Failure if Yarn Berry edges regress or order drifts
• pnpm test green
• DCO sign-off

Refs: #211 (pnpm fixture) · roadmap #144