feat: add Codecov coverage badge to README

[Hweinstock]

Closes #268

Problem

The CI pipeline uploads coverage to Codecov but the README has no coverage badge, making it hard to see coverage status at a glance.

Solution

Add the Codecov badge to the existing badge row in README.md.

Testing

Visual verification that badge renders correctly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@5aeca3c). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #356   +/-   ##
=======================================
  Coverage        ?   90.87%           
=======================================
  Files           ?       46           
  Lines           ?     4415           
  Branches        ?      673           
=======================================
  Hits            ?     4012           
  Misses          ?      233           
  Partials        ?      170           

Flag	Coverage Δ
unittests	90.87% <ø> (?)

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[Hweinstock]

Closing — Codecov integration needs to be enabled by a repo admin first before this badge is useful.