enhancement: add built-in FPM gcov bridge mode

[krystophny]

Category: enhancement
Priority: P2

PROBLEM: Users need an external script to bridge FPM build layouts to generate .gcov before analysis.

SOLUTION:

• Add a '--gcov' (or '--discover-and-gcov') mode:
  • Detect FPM build dirs with matching .gcno/.gcda
  • Run gcov in each dir (timestamp compatibility)
  • Collect/filter meaningful .gcov files and run analysis
• Use scripts/fpm_coverage_bridge.sh as reference