Fix code coverage: instrument the library and exclude third-party code

[K20shores]

Problem

Code coverage was reporting ~30%. Investigation found two issues, the second being the dominant one.

1. The library was never instrumented (root cause)

append_coverage_compiler_flags() was called after add_subdirectory(src). That function only appends to CMAKE_CXX_FLAGS, which affects targets defined after it runs — so the src/ library was compiled without -fprofile-arcs -ftest-coverage. It produced no .gcno/.gcda data, and the report covered only a handful of header lines inlined into the test translation units (lcov aggregated just 51 lines / 10 functions → 30% of functions).

Fix: move the coverage include() + append_coverage_compiler_flags() before add_subdirectory(src) so the library is instrumented.

2. Report included third-party / system code

The lcov capture only excluded test/*, so build/_deps/* (yaml-cpp, googletest, fmt) and inlined /usr/* headers would be counted once the library is instrumented.

Fix: add _deps/* and /usr/* to the lcov EXCLUDE list, plus a codecov.yml ignore: backstop.

Verification

docker build -t mc:cov -f docker/Dockerfile.coverage .
docker run --rm mc:cov bash -c 'make coverage ARGS="-j8"'

Before this change the lcov summary reported lines: 54.9% (28 of 51 lines) — i.e. it was measuring almost nothing. After instrumenting the library, the report should cover all of src/** and the public headers, and the percentage will reflect real coverage.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.13%. Comparing base (f3203c4) to head (55ec7e2).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #283       +/-   ##
===========================================
+ Coverage   70.00%   89.13%   +19.13%     
===========================================
  Files           5       37       +32     
  Lines          10     1611     +1601     
===========================================
+ Hits            7     1436     +1429     
- Misses          3      175      +172     

see 17 files with indirect coverage changes

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