Avoid non-API R lookup symbols under R-devel

[t-kalinowski]

Summary

Fix the R CMD check note on R-devel by removing compiled-code usage of R_NamespaceRegistry and R_UnboundValue.

Public-facing changes

There are no intended public-facing changes.

Internal changes

• replace direct namespace lookup with R_FindNamespace()
• replace lazy refenv lookups that used R_UnboundValue with nullable R_getVarEx(..., FALSE, NULL) lookups
• cache reticulate namespace bindings during init, including r_convert_dataframe_column and py_resolve_module_proxy
• add a compiled-code regression test that asserts the built library does not import R_NamespaceRegistry or R_UnboundValue