fix: ensure timespec.tv_nsec < 1_000_000_000 to meet POSIX constraints

[Appla]

rt

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.12%. Comparing base (6184d5c) to head (0affa15).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
src/os/timer.cc	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5865      +/-   ##
==========================================
+ Coverage   86.11%   86.12%   +0.01%     
==========================================
  Files         109      109              
  Lines       16820    16826       +6     
  Branches     2975     2977       +2     
==========================================
+ Hits        14485    14492       +7     
+ Misses       2335     2334       -1     

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