types: narrow BenchResult.engine to GraphEngine, cast topology in engine_factory

Replace BenchResult.engine: object | None with GraphEngine | None via
TYPE_CHECKING import (avoids circular import at runtime). Cast the
topology argument to TopologyGraph at the GraphEngine call site for
the duck-typed engine_factory. Fixes Pylance reportAttributeAccessIssue
warnings on op_log and topology arg. Type annotations only; no runtime
behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/kernbench/runtime_api/types.py

@@ -2,9 +2,13 @@ from __future__ import annotations
 
 import re
 from dataclasses import dataclass
+from typing import TYPE_CHECKING
 
 from kernbench.common.types import Completion, Trace
 
+if TYPE_CHECKING:
+    from kernbench.sim_engine.engine import GraphEngine
+
 
 @dataclass(frozen=True)
 class BenchResult:
@@ -12,7 +16,7 @@ class BenchResult:
     correlation_id: str
     trace: Trace | None = None
     traces: list[dict] | None = None
-    engine: object | None = None  # GraphEngine ref for Phase 2 data access
+    engine: GraphEngine | None = None
 
     def summary_text(self) -> str:
         if self.completion.ok: