docs: consolidate coverage workflow docs; link bridge script from README #1127
Description
Category: docs
Priority: P2
PROBLEM: Coverage workflow guidance is fragmented (README quick start says no manual gcov; detailed docs and FPM_COVERAGE_WORKFLOW_FIX.md instruct manual gcov and/or scripts). This confuses users when quick start fails.
SOLUTION:
- Consolidate coverage workflow in a single canonical doc under doc/ and link from README
- Clearly document two paths: (A) built-in auto-gcov (once implemented) and (B) current bridge script scripts/fpm_coverage_bridge.sh
- Ensure all examples are copy-paste runnable and consistent across README, CMake comments, and docs.