Rename graph/_utils to graph/_host_callback now that it holds only
host-callback machinery (the trampoline, _is_py_host_trampoline, and
_resolve_host_callback), matching the concept-named files around it, and
update the three cimport sites. Add _attach_host_callback_owners to share
the "callback -> slot 0, user_data -> slot 1" attachment between the eager
(GN_callback) and capture (add_callback) paths. Guard a zero-length
user_data copy against malloc(0) and hoist the per-call ctypes import.

Attach the kernel-argument tuple to the kernel node's slot 1 so the Python
objects backing the arguments -- notably device Buffers -- outlive the
graph. The driver copies argument values into the node at add time but does
not keep the referenced device memory alive, so without this a kernel node
could be left with a stale device pointer. This is the slot-table port of
the user-object fix from NVIDIA#2041 (currently only on main).

GraphNode.memcpy/memset (and the GraphDefinition pass-throughs) now accept a
Buffer or a raw int for each address. A new _resolve_ptr helper reads the
device pointer from a Buffer and returns it as an owner; a raw int casts
through with no owner. GN_memcpy attaches a Buffer dst to slot 0 and src to
slot 1, and GN_memset attaches dst to slot 0, so buffers passed by value
outlive the graph. Raw ints behave exactly as before (caller owns the
lifetime), so this is backward compatible.

Document the stream-capture lifetime contract on GraphBuilder: operations
recorded during capture reference caller-owned memory and are not retained,
unlike explicit GraphDefinition construction. Host callbacks are the one
exception, retained on both the capture and explicit paths.

… capture callbacks

Cover GraphDefinition memset/memcpy with Buffer operands (including clone),
and GraphBuilder capture host callbacks retained after dropping Python refs.

Keyword-only *_owner args retain arbitrary objects for raw pointer
operands; Buffer+owner combinations are rejected. Strengthen owner tests
with weakref retention checks and add src_owner rejection test.

Store DevicePtrHandle in slot table instead of Buffer wrappers so
reset/close cannot release memory while a graph still references it.
Add test-only weak_handle() for deterministic allocation lifetime
checks and extend graph lifetime tests accordingly.