Split _graphdef.pyx into _graph_def/ subpackage for maintainability

The monolithic _graphdef.pyx (2000+ lines) is split into three focused
modules under _graph_def/: _graph_def.pyx (Condition, GraphAllocOptions,
GraphDef), _graph_node.pyx (GraphNode base class and builder methods),
and _subclasses.pyx (all concrete node subclasses). Long method bodies
in GraphNode are factored into cdef inline GN_* helpers following
existing codebase conventions. Handle property annotations updated to
use driver.* types consistently.

Made-with: Cursor

Author: Andy-Jost

cuda_core/cuda/core/_graph/_graph_def/__init__.pxd

@@ -0,0 +1,23 @@
+# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+from cuda.core._graph._graph_def._graph_def cimport Condition, GraphDef
+from cuda.core._graph._graph_def._graph_node cimport GraphNode
+from cuda.core._graph._graph_def._subclasses cimport (
+    AllocNode,
+    ChildGraphNode,
+    ConditionalNode,
+    EmptyNode,
+    EventRecordNode,
+    EventWaitNode,
+    FreeNode,
+    HostCallbackNode,
+    IfElseNode,
+    IfNode,
+    KernelNode,
+    MemcpyNode,
+    MemsetNode,
+    SwitchNode,
+    WhileNode,
+)

cuda_core/cuda/core/_graph/_graph_def/__init__.py

@@ -0,0 +1,51 @@
+# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+"""Explicit CUDA graph construction — GraphDef, GraphNode, and node subclasses."""
+
+from cuda.core._graph._graph_def._graph_def import (
+    Condition,
+    GraphAllocOptions,
+    GraphDef,
+)
+from cuda.core._graph._graph_def._graph_node import GraphNode
+from cuda.core._graph._graph_def._subclasses import (
+    AllocNode,
+    ChildGraphNode,
+    ConditionalNode,
+    EmptyNode,
+    EventRecordNode,
+    EventWaitNode,
+    FreeNode,
+    HostCallbackNode,
+    IfElseNode,
+    IfNode,
+    KernelNode,
+    MemcpyNode,
+    MemsetNode,
+    SwitchNode,
+    WhileNode,
+)
+
+__all__ = [
+    "AllocNode",
+    "ChildGraphNode",
+    "Condition",
+    "ConditionalNode",
+    "EmptyNode",
+    "EventRecordNode",
+    "EventWaitNode",
+    "FreeNode",
+    "GraphAllocOptions",
+    "GraphDef",
+    "GraphNode",
+    "HostCallbackNode",
+    "IfElseNode",
+    "IfNode",
+    "KernelNode",
+    "MemcpyNode",
+    "MemsetNode",
+    "SwitchNode",
+    "WhileNode",
+]

cuda_core/cuda/core/_graph/_graph_def/_graph_def.pxd

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+# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+from cuda.bindings cimport cydriver
+from cuda.core._resource_handles cimport GraphHandle
+
+
+cdef class Condition:
+    cdef:
+        cydriver.CUgraphConditionalHandle _c_handle
+        object __weakref__
+
+
+cdef class GraphDef:
+    cdef:
+        GraphHandle _h_graph
+        object __weakref__
+
+    @staticmethod
+    cdef GraphDef _from_handle(GraphHandle h_graph)