Add accessor methods to shared_resource<T>

[bdice]

Feature Request: Add accessor methods to shared_resource<T>

Problem

shared_resource<T> currently provides no way to access the underlying T object. Unlike shared_ptr<T> which offers operator-> and .get(), shared_resource<T> has no equivalent accessors.

Motivating Use Case

In the RMM Python/Cython layer, a Cython object like StatisticsResourceAdaptor holds a C++ member c_obj of type shared_resource<statistics_resource_adaptor>.

• Shared ownership is required because the underlying memory resource is stateful
• Access to methods of statistics_resource_adaptor is needed to retrieve current statistics

Without accessors, the RMM Python/Cython layer cannot be cleanly implemented.

Proposed Solution

Add smart-pointer-like accessors:

Method	Return Type	Description
operator->	T*	Pointer access to underlying resource
operator*	T&	Reference to underlying resource
.get()	T&	Named accessor returning reference

Note: .get() returns T& rather than T* (unlike shared_ptr::get()) because shared_resource cannot be in a null state.

Alternatives Considered

• T& underlying_resource(): A single named accessor. Rejected in favor of the more familiar smart-pointer interface, though functionally equivalent.

Additional Context

We would like this feature to be backported to branch/3.2.x for adoption in RMM 26.04.