"simple" formatter outputs immediately

[forman]

[Description of PR]

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

Attention: Patch coverage is 98.05825% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.14%. Comparing base (2ccfc58) to head (8079df5).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
xrlint/cli/engine.py	92.30%	1 Missing ⚠️
xrlint/formatters/simple.py	98.07%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   95.95%   96.14%   +0.18%     
==========================================
  Files          51       52       +1     
  Lines        1732     1765      +33     
  Branches      232      236       +4     
==========================================
+ Hits         1662     1697      +35     
+ Misses         36       34       -2     
  Partials       34       34              

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