Add backports module for R 4.5 C-API compliance

[CGMossa]

TODO: replace this line with your own framing, in your own voice.

AI-written details

Summary

• R's C API is stabilizing: in R 4.5 several entry points (ENCLOS, Rf_findVar, Rf_findVarInFrame, CLOENV, BODY, FORMALS, DATAPTR, Rf_isFrame) became non-API and were replaced by compliant functions (R_ParentEnv, R_getVar, R_getVarEx, R_ClosureEnv, R_ClosureBody, R_ClosureFormals, DATAPTR_RO, Rf_isDataFrame). Code that targets both old and new R has to branch per version.
• Ports extendr-ffi's backports module verbatim (with attribution) into src/backports.rs: small unsafe fn wrappers (get_parent_env, get_var, get_var_safe, get_var_in_frame, get_closure_env/body/formals, dataptr, is_data_frame) that dispatch to the correct entry point per R version.
• The module declares its own cfg-gated extern block, so it does not depend on which symbols the committed bindings happen to include.
• build.rs now emits r_4_4 / r_4_5 cfg flags (with matching rustc-check-cfg) for the module to gate on. These are internal to this crate.

Test plan

• cargo build against R 4.6.0 (so r_4_5 is set): compiles, 0 errors.
• Linkage check (run locally, not committed): calling backports::get_parent_env(R_GlobalEnv) and backports::is_data_frame(...) from the existing embedded-R test resolves and runs against R 4.6.0.
• Build against an R < 4.5 (so the not(r_4_5) branch compiles and links the old entry points).

Notes

• Copied rather than depended on, since libR-sys cannot depend on extendr-ffi.
• Other extendr-ffi modules were considered and not copied: lib.rs/altrep.rs/graphics.rs/symbols.rs are a hand-curated subset of what libR-sys already generates; the idiomatic enums overlap with the existing Rboolean enum and the convert_to_rust_enums work; non_api.rs could follow later behind a feature if wanted.

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Drafted by Claude (claude-opus-4-8). Reviewed by the author.