Add HC1/HC2/HC3 estimators, improve tests and docs

[matthieugomez]

Summary

• Add Vcov.hc(n) constructor for HC1, HC2, HC3 heteroskedasticity-robust variants; robust() remains HC1 for backward compatibility
• HC2/HC3 use leverage-adjusted residuals (OLS only)
• Switch from manual Base.precompile hints to PrecompileTools @compile_workload
• Add vcov computation tests verified against manual formulas (Simple, Robust HC1, HC2, HC3, Cluster, ranktest)
• Fix README: replace nonexistent dof_tstat with dof_residual, document all estimators
• Minor cleanup: || in ranktest, remove unused import, simplify S_hat broadcasting
• Bump version to 0.8.3

Test plan

• All 36 tests pass locally (Pkg.test())
• CI passes on GitHub Actions

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.91667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (ed81967) to head (a1abbd5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/covarianceestimators/vcovrobust.jl	97.05%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #27       +/-   ##
===========================================
+ Coverage   28.57%   92.26%   +63.69%     
===========================================
  Files           7        7               
  Lines         140      168       +28     
===========================================
+ Hits           40      155      +115     
+ Misses        100       13       -87     

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