Add Codecov badge to README

[Copilot]

Adds a Codecov coverage badge to the README tracking the main branch.

The badge displays current coverage percentage and links to the full Codecov dashboard:

[![codecov](https://codecov.io/gh/nanonyme/simplecpreprocessor/branch/main/graph/badge.svg)](https://codecov.io/gh/nanonyme/simplecpreprocessor)

Positioned below the title where badges are conventionally placed.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add codecov badge to Readme</issue_title>
<issue_description>This can be always for main.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add codecov badge to Readme #93

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.90%. Comparing base (ed0673d) to head (801e4a3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   97.90%   97.90%           
=======================================
  Files           7        7           
  Lines         763      763           
  Branches      160      160           
=======================================
  Hits          747      747           
  Misses         10       10           
  Partials        6        6           

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