Add session-scoped topology fixture in tests/conftest.py

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>

tests/conftest.py

@@ -0,0 +1,36 @@
+"""Shared pytest fixtures for the kernbench test suite.
+
+Session-scoped topology caching: ``resolve_topology("topology.yaml")`` is
+pure (no side effects), so we cache the result across all tests in a
+worker process. Each test still builds its own ``GraphEngine`` (which is
+stateful/SimPy-event-consuming and MUST NOT be shared).
+"""
+from __future__ import annotations
+
+import pytest
+
+from kernbench.topology.builder import resolve_topology
+
+
+@pytest.fixture(scope="session")
+def topology():
+    """Session-scoped parsed topology (immutable graph + spec).
+
+    Usage in tests::
+
+        def test_foo(topology):
+            engine = GraphEngine(topology.topology_obj, enable_data=True)
+    """
+    return resolve_topology("topology.yaml")
+
+
+@pytest.fixture(scope="session")
+def topology_obj(topology):
+    """The TopologyGraph inside the handle (convenience shortcut)."""
+    return topology.topology_obj
+
+
+@pytest.fixture(scope="session")
+def spec(topology):
+    """Topology spec dict (convenience shortcut)."""
+    return topology.topology_obj.spec