fix: update nyc from 15.1.0 to 18.0.0

Per kadel's feedback: "latest version is 18.0.0, 15.1.0 is 6 years old"

Updated nyc from 15.1.0 (2020) to 18.0.0 (latest, Feb 2026) in:
- scripts/instrument-plugin.sh
- scripts/report-coverage.sh

Ensures we get latest Istanbul features and bug fixes.

Author: gustavolira

scripts/instrument-plugin.sh

@@ -130,7 +130,7 @@ while IFS= read -r PROD_IMAGE; do
   # Instrument with nyc (pinned version for reproducibility)
   # Must run from work directory to avoid "outside project root" errors
   echo "  Instrumenting with Istanbul/nyc..."
-  if ! (cd "$WORK_DIR" && npx --yes nyc@15.1.0 instrument dist-original dist-instrumented --source-map); then
+  if ! (cd "$WORK_DIR" && npx --yes nyc@18.0.0 instrument dist-original dist-instrumented --source-map); then
     echo "  ❌ Instrumentation failed - skipping"
     rm -rf "$WORK_DIR"
     ((SKIPPED_COUNT++))

scripts/report-coverage.sh

@@ -39,8 +39,8 @@ fi
 echo ""
 echo "[INFO] Merging coverage data with nyc..."
 mkdir -p "$REPO_ROOT/.nyc_output"
-npx nyc@15.1.0 merge "$REPO_ROOT/$COVERAGE_JSON_DIR" "$REPO_ROOT/.nyc_output/out.json"
-(cd "$REPO_ROOT" && npx nyc@15.1.0 report --reporter=lcov --reporter=text-summary --report-dir coverage)
+npx nyc@18.0.0 merge "$REPO_ROOT/$COVERAGE_JSON_DIR" "$REPO_ROOT/.nyc_output/out.json"
+(cd "$REPO_ROOT" && npx nyc@18.0.0 report --reporter=lcov --reporter=text-summary --report-dir coverage)
 
 if [[ ${#WORKSPACES[@]} -gt 1 ]]; then
   echo "[WARN] Multi-workspace coverage upload is not supported." >&2