Add Codecov badge and enforce coverage thresholds

[jose-compu]

Description

The CI workflow has a coverage job using cargo llvm-cov with Codecov upload, but fail_ci_if_error: false means coverage failures are silently ignored. There is also no Codecov badge in the README.

Proposed Changes

• Add Codecov badge to README.md
• Configure codecov.yml with:
  • Minimum coverage thresholds (e.g., 70% project, 50% patch)
  • Status checks on PRs
• Set fail_ci_if_error: true once Codecov token is confirmed working

Impact

Visible coverage metrics and prevents coverage regressions.