refactor: use external tests and add t.Parallel()

[mpyw]

Summary

• Convert internal tests to external test packages (foo_test format)
• Add t.Parallel() to enable parallel test execution
• Create export_test.go files to expose unexported functions for testing

Files Changed

• internal/probe/composite_test.go
• internal/directive/carrier/carrier_test.go

Test plan

• go test ./... passes locally
• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.25%. Comparing base (1869c98) to head (1934652).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   84.25%   84.25%           
=======================================
  Files          24       24           
  Lines        1842     1842           
=======================================
  Hits         1552     1552           
  Misses        173      173           
  Partials      117      117           

Flag	Coverage Δ
unittests	84.25% <ø> (ø)

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