Add a strict Buffer Resource back-reference to Host and Pinned memory resources

[nirandaperera]

HostMemoryResource and PinnedMemoryResource are now BufferResource-only resources that carry a strict BackRefMixin<BufferResource> back-reference. When a copy of one of these resources is made (e.g. when CCCL promotes host_mr() / pinned_mr() into an owning cuda::mr::any_resource inside a HostBuffer), the copy promotes the stored weak_ptr to a shared_ptr, keeping the owning BufferResource alive for as long as any derived buffer lives. This closes a lifetime gap where a Buffer could outlive the BufferResource that produced its memory resource.

The mixin is strict: copying an instance without an installed back-reference throws std::bad_weak_ptr. To make that safe, these resources can no longer be constructed as standalone objects — they are created only by BufferResource, which installs the back-reference immediately after construction. This mirrors how RmmResourceAdaptor already works on this branch.

C++ changes

• HostMemoryResource: inherits BackRefMixin<BufferResource>; constructor is private with friend class BufferResource. Equality remains stateless (always true) since instances are interchangeable and the back-reference is installed exactly once.
• PinnedMemoryResource: inherits BackRefMixin<BufferResource> as a second base; constructor is private (friend class BufferResource). The make_if_available / from_options factories were removed so no back-reference-less instance can ever escape. Equality compares the shared pool state only.
• PinnedPoolProperties: gained a numa_id field (defaults to the calling thread's NUMA node) so all pinned configuration flows as one struct. Added a free helper pinned_pool_properties_from_options(...).
• PinnedMemoryDisabled: new constexpr std::nullopt_t sentinel used to disable pinned host memory (replaces the old PinnedMemoryResource::Disabled).
• BufferResource::create() / from_options(): now take std::optional<PinnedPoolProperties> pinned_pool_properties (default PinnedMemoryDisabled) instead of a pre-built resource. The pinned resource is constructed internally and the back-reference is installed on owning_mr_, host_mr_, and pinned_mr_ before create() returns.
• try_pinned_mr(): now returns std::optional<PinnedMemoryResource> (a back-referenced handle) so Python/Statistics get a concrete, lifetime-safe handle.

Python changes

• PinnedMemoryResource: now an opaque, non-constructible handle (__init__ raises TypeError); obtained via BufferResource.pinned_mr.
• PinnedPoolProperties: new @dataclass (initial_pool_size, max_pool_size, numa_id) used to configure pinned memory.
• BufferResource(...): replaces the old pinned_mr= argument with pinned_pool_properties: PinnedPoolProperties | None = None (None disables pinned host memory). from_options derives this from the config options.
• Statistics.report(pinned_mr=...): unchanged signature; the handle is now sourced from BufferResource.pinned_mr.

Tests

• Added HostMrKeepsBufferResourceAlive and PinnedMrKeepsBufferResourceAlive regression tests mirroring DeviceMrKeepsBufferResourceAlive.
• Routed all C++ tests/benchmarks through a BufferResource (no standalone Host/Pinned MR construction); reworked test_host_buffer, test_config, test_memory_resources, and bench_memory_resources.
• Updated Python test_config.py to configure pinned memory via BufferResource and assert on BufferResource.pinned_mr.

Breaking changes

• C++: HostMemoryResource / PinnedMemoryResource can no longer be constructed directly; PinnedMemoryResource::make_if_available / from_options and the Disabled sentinel are removed. BufferResource::create() takes PinnedPoolProperties instead of a PinnedMemoryResource.
• Python: PinnedMemoryResource(...) is no longer constructible; the BufferResource(pinned_mr=...) argument is replaced by pinned_pool_properties=.

Depends on #1078

Closes #1070

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[madsbk]

Overall looks good

[wence-]

Code changes broadly look good, but please clean up the docstrings and comments throughout.

In general, please try and elide as much implementation detail in comments and, if it is necessary, have it only in one place rather than everywhere. Docstrings and comments should generally talk about observable behaviour and semantics, not implementation specifics.

[madsbk]

Looks good. The only thing we should consider is the enablement semantics. Currently, setting pinned_pool_properties to a non-None value does not guarantee that pinned memory is enabled, since that still depends on system support.

It would be helpful to have an option that means: enable pinned memory, or raise an error if it is unavailable.

Alternatively, this option could be implemented at the cudf-polars level. After creating the memory resource, cudf-polars could verify that pinned memory was actually enabled and raise an error otherwise.

[nirandaperera]

Looks good. The only thing we should consider is the enablement semantics. Currently, setting pinned_pool_properties to a non-None value does not guarantee that pinned memory is enabled, since that still depends on system support.

It would be helpful to have an option that means: enable pinned memory, or raise an error if it is unavailable.

Alternatively, this option could be implemented at the cudf-polars level. After creating the memory resource, cudf-polars could verify that pinned memory was actually enabled and raise an error otherwise.

I made BufferResource::create throw, if PinnedProperties provided and pinned memory is not supported

[nirandaperera]

/merge

dismissing request as all comments have been addressed