[RFC 0005] Persist C++ Samples in Model Objects for Continued From-Memory Sampling

[andrewherren]

• R/Python model object holds a persistent pointer to the C++ BARTSamples / BCFSamples object.
• Also retain the BARTSampler / BCFSampler (RNG state + data refs) for the model's lifetime; one sampler state per chain for multi-chain.
• Lazy unpack: only materialize parameter arrays into R/Python when queried via extract_parameter.
• Deprecation shim for moved fields (e.g. global_var_samples, leaf_scale_samples):
  • Python __getattr__ maps moved attr → extract_parameter(key), emits DeprecationWarning.
  • R $.bartmodel (and $.bcfmodel) overload maps moved field → extract_parameter, emits warning.
  • Document the shim is for one deprecation cycle; later release raises (Python) / returns NULL (R).