feat: add optional .threads control across fit/cv/tune

[ielbadisy]

Summary

• add optional argument to , , and
• set Torch CPU threads via when provided
• propagate through nested cross-validation and tuning fits
• include thread setting in training output
• update README, NEWS, and cran-comments for resubmission
• regenerate Rd docs and update tests

Validation

• R CMD build .
• R CMD check --no-manual --run-donttest survdnn_0.7.6.tar.gz (Status: OK)
• R CMD INSTALL survdnn_0.7.6.tar.gz (Status: OK)