Add tests for visualization/ and metrics/, then raise coverage threshold to 60%

## Context

CI enforces `--cov-fail-under=40` (`pyproject.toml`), while actual coverage sits around 55%. The gap is almost entirely in `src/lanet_vi/visualization/` (layout, matplotlib renderer) and `src/lanet_vi/metrics/`, which have no direct tests.

## Plan

1. Add tests for `visualization/` — layout coordinates for a small known graph, renderer smoke tests on a headless `Agg` backend, legend content (this would also catch #10).
2. Add tests for `metrics/`.
3. Raise `--cov-fail-under` to 60 once the suite is there. Do not raise the threshold first — that just makes CI red without adding coverage.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start with the existing test layout and the --cov-fail-under=40 setting in pyproject.toml, then inspect src/lanet_vi/visualization/ and src/lanet_vi/metrics/. Run the current test suite with coverage to establish the baseline. Done means direct tests cover the listed layout, headless renderer, legend, and metrics behavior, and CI passes with the threshold raised to 60%.

Written by the indexing model from the issue text.