Test: add Poetry (poetry.lock) edge-case fixture for vg build

[vibgrate-team]

Context

We already seed fixtures for pnpm, Yarn Berry, Cargo workspaces, and Go workspaces. Poetry (pyproject.toml + poetry.lock) is a common Python path that still needs a small deterministic corpus so vg build / dependency graph extraction stays stable.

What to do

• Add a minimal fixture under test/ (or the repo’s existing fixture layout) with a Poetry lock that includes at least one path/git or extras edge case if supported — keep it tiny.
• Assert deterministic graph.json (stable IDs/sorts) for vg build on that fixture.
• Follow CONTRIBUTING grammar/fixture patterns; no network during the test.

Acceptance

• Fixture is checked in and builds offline
• Test asserts stable graph output
• No secrets, no live registry calls
• DCO sign-off (git commit -s)