enhancement: add CI E2E validation vs pycobertura markdown

[krystophny]

Category: enhancement
Priority: P0

PROBLEM: No automated end-to-end validation of FortCov’s markdown against a trusted baseline (pycobertura). Regressions can slip (see file-vs-line counting bug).

SOLUTION:

• New CI job:
  1. fpm test --flag "-fprofile-arcs -ftest-coverage"
  2. Generate Cobertura XML via gcovr from build artifacts
  3. Render markdown via pycobertura (or HTML→markdown if markdown unsupported)
  4. Run fortcov to produce coverage.md
  5. Compare key metrics (line rate, totals per file) and fail on deltas
  6. Upload both artifacts
• Capture tool versions; 300s cap.