fix(r): .onAttach/.onLoad crash on NULL kit.nThread

[davidbudzynski]

Closes #56.

Reattach with options(kit.nThread=NULL) failed: .onAttach did if (nth > 1L) on NULL (logical(0) -> 'argument is of length zero'); NA failed with 'missing value where TRUE/FALSE needed'; length>1 failed similarly.

• R/call.R .onAttach: nth = getOption("kit.nThread", 1L) + if (is.null(nth) || length(nth) != 1L || is.na(nth)) nth = 1L; startup message only when interactive(). Keeps CompEnabledR naming.
• .onLoad: reset default via is.null(getOption("kit.nThread")) instead of names() check.
• test-nthread-opt.R (10 tests): .onLoad restore/preserve, .onAttach with NULL/NA/0/"2"/c(1L,2L), iif/vswitch with NULL default. Runs with OpenMP disabled (direct calls, no detach).
• NEWS.md: bug-fix entry.

Verification: all 5 bad values attach clean, .onLoad restores 1L, Rscript startup silent, full tinytest suite green (1409 results).

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (99413b6) to head (4953cae).
⚠️ Report is 63 commits behind head on master.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
+ Coverage   98.01%   98.27%   +0.26%     
==========================================
  Files          13       13              
  Lines        5083     5100      +17     
==========================================
+ Hits         4982     5012      +30     
+ Misses        101       88      -13     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.