docs: add Codacy grade badge to README

[petercorke]

Adds the Codacy code-quality grade badge to the Status & Project Health group, placed after Coverage as the rvc-ecosystem badge order specifies. This matches RTB, MVTB and bdsim.

The current grade is B (74). The ecosystem target is A.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.05%. Comparing base (5902a19) to head (e457423).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   85.05%   85.05%           
=======================================
  Files           2        2           
  Lines         776      776           
=======================================
  Hits          660      660           
  Misses        116      116           

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