Provides a shared `topology` fixture that caches the parsed
topology.yaml result per pytest-xdist worker session. Tests that
build a GraphEngine can accept `topology` instead of calling
resolve_topology("topology.yaml") repeatedly.
Topology parsing costs ~32ms, so the practical saving per worker is
modest (<1s across all tests). The fixture is mainly for architectural
cleanliness — keeping the "parse once, build engine many" pattern
explicit.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
