test: add default causal recovery accuracy check

[chaewon-research]

Fixes #15

Summary

• Adds a lightweight ground-truth causal-recovery accuracy check to the default test path (test_discover_network_basic_numpy).
• Asserts recovery of the true X0 -> X1 edge at lag 1 and verifies no false edges are discovered.
• Keeps the broader integration recovery suite behind the existing integration marker so CI stays fast.

Why

The existing integration recovery tests are excluded by default in pyproject.toml, allowing local test runs and CI to pass without checking whether causal recovery actually works.

Validation

• Focused test passes in ~0.12s:
  python -m pytest causationentropy/tests/test_discovery.py::TestDiscoverNetwork::test_discover_network_basic_numpy -v
• Full default test suite passes: 381 passed, 17 deselected in ~25s.
• black, isort, and flake8 checks all pass cleanly.