bug: README promises auto-gcov but tool requires pre-generated .gcov

[krystophny]

Category: bug
Priority: P0

PROBLEM: README "FPM Quick Start" says fortcov auto-discovers and runs gcov (no manual steps). Actual behavior requires .gcov files; fortcov does not invoke gcov.

EVIDENCE:

• After removing .gcov files and running fortcov: "No coverage files found... To generate coverage files... gcov ..."
• Repro:
  rm -f build/**/.gcov; fpm test --flag "-fprofile-arcs -ftest-coverage"; build/gfortran_/app/fortcov --output=coverage.md

SOLUTION:

• Implement built-in gcov invocation for FPM builds (discover .gcno/.gcda, run gcov per dir) OR
• Update README to reflect current workflow and point to scripts/fpm_coverage_bridge.sh.
• Add an integration test that starts from no .gcov and verifies end-to-end generation.

[krystophny]

Resolved by PR #1133