Fix root logging level being set to INFO for all packages

[PauBadiaM]

Summary

• Replaced logging.basicConfig() in _log.py with a named decoupler logger so importing decoupler no longer sets the root logging level to INFO for all packages
• Updated tests to capture logs from the named logger

Closes #296

Test plan

• Verified root logger is unaffected after import decoupler
• All 31 tests in tests/pp/test_net.py and tests/ds/test_toy.py pass
• Pre-commit checks pass

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.31%. Comparing base (40092de) to head (24badca).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #298   +/-   ##
=======================================
  Coverage   94.30%   94.31%           
=======================================
  Files          78       78           
  Lines        4075     4079    +4     
=======================================
+ Hits         3843     3847    +4     
  Misses        232      232           

Files with missing lines	Coverage Δ
src/decoupler/_log.py	100.00% <100.00%> (ø)

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