docs: Add CI, crates.io, and codecov badges to README

[scouten-adobe]

Summary

• Add CI, crates.io Latest Version, and codecov badges to the top of the README, matching the style used in the c2pa-rs root README.
• Docs.rs badge is intentionally omitted since c2patool isn't published there.

Test plan

• Rendered README locally to confirm badge Markdown is well-formed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.16%. Comparing base (5ea32f5) to head (2284011).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #312   +/-   ##
=======================================
  Coverage   72.16%   72.16%           
=======================================
  Files           4        4           
  Lines        1060     1060           
=======================================
  Hits          765      765           
  Misses        295      295           

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