Add Codecov badge to README (#94)

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:

```markdown
[![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.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *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)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #93

<!-- START COPILOT CODING AGENT TIPS -->
---

✨ Let Copilot coding agent [set things up for
you](https://github.com/nanonyme/simplecpreprocessor/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot)
— coding agent works faster and does higher quality work when set up for
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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: nanonyme <348449+nanonyme@users.noreply.github.com>

Author: Copilot

README.md

@@ -1,5 +1,7 @@
 # simplepreprocessor
 
+[![codecov](https://codecov.io/gh/nanonyme/simplecpreprocessor/branch/main/graph/badge.svg)](https://codecov.io/gh/nanonyme/simplecpreprocessor)
+
 Usage
 ---------