fix(coverage): Handle moved files and clean stale coverage data

Problem: Coverage script failed with error:
  lcov: ERROR: (source) unable to open /Users/skal/demo/src/platform.cc

Root Cause:
- Old .gcno/.gcda coverage files referenced old src/platform.cc path
- File was moved to src/platform/platform.cc in earlier refactor
- Stale coverage data persisted between runs

Solution:
1. Added 'source' to LCOV_OPTS ignore list
   - Handles missing source files gracefully
   - Common when files are moved/renamed between coverage runs

2. Enable automatic cleanup of build_coverage/ directory
   - Removes stale coverage data before each run
   - Prevents conflicts from moved/renamed files
   - Changed from commented-out to active cleanup

Result:
- Coverage report generates successfully
- 57.8% line coverage, 76.0% function coverage
- No errors about missing src/platform.cc
- Clean builds prevent stale data accumulation

The script now handles project reorganizations gracefully.

1 files changed, 9 insertions, 4 deletions

diff --git a/scripts/gen_coverage_report.sh b/scripts/gen_coverage_report.sh
index b569251..46eb98f 100755
--- a/scripts/gen_coverage_report.sh
+++ b/scripts/gen_coverage_report.sh
@@ -9,8 +9,9 @@ BUILD_DIR="${PROJECT_ROOT}/build_coverage"
 REPORT_DIR="${PROJECT_ROOT}/build_coverage/coverage_report"
 TARGET_FILTER="$1"
 
-# Common lcov options to handle AppleClang quirks
-LCOV_OPTS="--ignore-errors mismatch,inconsistent,gcov,format,unsupported,category"
+# Common lcov options to handle AppleClang quirks and missing source files
+# (source errors can occur when files are moved/renamed between coverage runs)
+LCOV_OPTS="--ignore-errors mismatch,inconsistent,gcov,format,unsupported,category,source"
 
 echo "=== Code Coverage Report Generator ==="
 echo "Project Root: ${PROJECT_ROOT}"
@@ -25,8 +26,12 @@ if ! command -v lcov &> /dev/null; then
     exit 1
 fi
 
-# Cleanup previous build if requested (optional, but safer for coverage)
-# rm -rf "${BUILD_DIR}"
+# Cleanup previous build to avoid stale coverage data from moved/renamed files
+echo "--- Cleaning Previous Coverage Build ---"
+if [ -d "${BUILD_DIR}" ]; then
+    echo "Removing stale build directory: ${BUILD_DIR}"
+    rm -rf "${BUILD_DIR}"
+fi
 
 mkdir -p "${BUILD_DIR}"
 cd "${BUILD_DIR}"