CI: track all instrumented lines in coverage report (--process-gcno)

fastcov was only processing .gcda files, which are created at runtime.
Source files compiled with --coverage but never executed by tests (most
of applications/ and tools/) had .gcno but no .gcda, so they were silently
omitted from the report.

Adding --process-gcno makes fastcov include .gcno-only files at 0%
coverage, so ALL instrumented lines across libraries, applications, and
tools are tracked.  Expected tracked lines: ~377K (was ~67.5K).

Author: chdinh

.github/workflows/_reusable-tests.yml

@@ -207,8 +207,12 @@ jobs:
         echo "=== Coverage diagnostics ==="
         echo "gcno files: $(find build -name '*.gcno' 2>/dev/null | wc -l)"
         echo "gcda files: $(find build -name '*.gcda' 2>/dev/null | wc -l)"
-        # Collect coverage data with fastcov (10-100x faster than lcov)
-        fastcov -d build -o coverage.json
+        # Collect coverage data with fastcov (10-100x faster than lcov).
+        # --process-gcno includes .gcno files without .gcda counterparts
+        # so that untested source files appear in the report at 0% instead
+        # of being silently omitted.  This tracks ALL instrumented lines
+        # across libraries, applications, and tools.
+        fastcov --process-gcno -d build -o coverage.json
         # Filtered report: MNE-CPP libraries, applications, and tools
         fastcov -C coverage.json --lcov -o coverage_filtered.info \
              --include src/libraries/ src/applications/ src/tools/ \