Remove unprotect calls that rchk claims to be over unprotecting.

[astamm]

While checking against rhub/rchk the {phutil} package I am about to submit to CRAN, it spotted:

Function cpp11::detail::store::insert(SEXPREC*)
  [PB] has negative depth /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/protect.hpp:302
  [UP] attempt to unprotect more items (2) than protected (0), results will be incomplete /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/protect.hpp:302
  [PB] has possible protection stack imbalance /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/protect.hpp:305

Function cpp11::writable::r_vector<double>::reserve_data(SEXPREC*, bool, long)
  [PB] has negative depth /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1342
  [UP] attempt to unprotect more items (2) than protected (0), results will be incomplete /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1342
  [PB] has possible protection stack imbalance /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1343

Function cpp11::writable::r_vector<double>::resize_data(SEXPREC*, bool, long)
  [PB] has negative depth /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1367
  [UP] attempt to unprotect more items (1) than protected (0), results will be incomplete /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1367
  [PB] has possible protection stack imbalance /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1368

Function cpp11::writable::r_vector<double>::resize_names(SEXPREC*, long)
  [PB] has negative depth /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1389
  [UP] attempt to unprotect more items (1) than protected (0), results will be incomplete /github/home/R/x86_64-pc-linux-gnu-library/4.6/cpp11/include/cpp11/r_vector.hpp:1389

Full details here: https://github.com/tdaverse/phutil/actions/runs/14893316603/job/41830462913.

I therefore temporarily vendored {cpp11} headers and removed the UNPROTECT() calls and now rchk does not complain anymore.

This PR proposes their removal but maybe it needs more tests/investigation.

[astamm]

Though it seems to break the CI because FAQ vignette fails with these modifications.
Maybe it was a false positive from rchk.
If you can shed some light on this, I would very much appreciate it. Thanks.

[DavisVaughan]

These unprotects look correct to me. You can see the matching PROTECT() above them.

[astamm]

Yes, but still rchk complains.

I found a fix for insert in protect.hpp.

The key changes were to remove PROTECT/UNPROTECT calls since:

• list from get() is already preserved;
• The cons cell is part of the preserve list structure;
• The input x is protected by the caller.

I have not yet managed to find the issue in r_vector.hpp.

[astamm]

I found the fix for r_vector.hpp. In summary,

Key changes to reserve_data()

1. Removed all explicit PROTECT/UNPROTECT calls since protection is handled by the safe wrapper in both resize_data and resize_names()
2. Simplified the flow of the names handling
3. Let R's garbage collection handle the intermediate objects

Key change to resize_data()

1. Moving both the PROTECT and UNPROTECT calls since the safe wrapper around Rf_allocVector already handles protection.

Key changes to resize_names()

1. The safe wrapper around Rf_allocVector already handles protection
2. Removed direct pointer access via STRING_PTR_RO
3. Use STRING_ELT instead of direct pointer access

[DavisVaughan]

The safe wrapper does not handle protection when the output type is SEXP. The caller must still protect the SEXP output. There must be a problem with rchk, possibly only with r-devel, the existing code looks fine to me.