Fix false Codecov project coverage drops by uploading R and JS reports together (#254)

[Nishita-shah1]

Fixes #254

Summary

• Codecov project was failing on documentation-only commits because R and JS coverage were uploaded separately. If the JS job failed, an R-only report became the baseline, so the next full R+JS upload looked like a coverage drop.
• Coverage jobs now save artifacts instead of uploading immediately. A follow-up job uploads both reports together, and only if both R and JS jobs succeed.
• CRAN checks stay in their own job. Texlive is installed only there.

Test plan

• Confirm CI runs R_coverage and JS_coverage in parallel, then Upload combined coverage to Codecov.
• Confirm the combined upload job is skipped if either coverage job fails.
• Confirm CRAN still runs independently.
• On the PR, Codecov patch and project should not fail just because one coverage job failed on a previous commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.79%. Comparing base (f265a03) to head (c923918).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
- Coverage   72.88%   72.79%   -0.09%     
==========================================
  Files         164      164              
  Lines        8868     9429     +561     
==========================================
+ Hits         6463     6864     +401     
- Misses       2405     2565     +160     

Flag	Coverage Δ
javascript	?
r	?

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[tdhock]

@ANAMASGARD please review?