fix: windows core count from powershell

[maxnbk]

Closes #2029 .

Explanation:

wmic was removed as a default install in Windows 11 24H2, causing rez to fail at startup when physical core count is computed.

Fix:

Replace it with a PowerShell/Get-CimInstance equivalent - powershell.exe (Windows PowerShell 5.1) ships with all Windows 10/11 installs and is guaranteed available. The old wmic path is retained as a fallback for older Windows builds.

Disclosure:

This PR was AI-assisted with Claude Code, Sonnet 4.6, primarily for diagnostics, logic-tracing, documentation and edge-case verification.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.69%. Comparing base (d415b96) to head (f92b52e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2108      +/-   ##
==========================================
+ Coverage   60.65%   60.69%   +0.04%     
==========================================
  Files         164      164              
  Lines       20584    20598      +14     
  Branches     3579     3580       +1     
==========================================
+ Hits        12485    12502      +17     
+ Misses       7224     7220       -4     
- Partials      875      876       +1     

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