test: expand reporter and scan coverage

[ColumbusLabs]

Summary

• add JSON reporter contract coverage
• add scan() integration coverage for examples/react
• expand CI smoke scans to React Native and Next examples

Closes #16
Closes #17
Closes #28

Validation

• npm run typecheck
• npm run typecheck:tests
• npm test
• npm run build
• node dist/cli/index.js scan examples/react --min-severity info --format markdown --output /tmp/debtlens-react.md
• node dist/cli/index.js scan examples/react-native --min-severity info --format markdown --output /tmp/debtlens-react-native.md
• node dist/cli/index.js scan examples/next --min-severity info --format markdown --output /tmp/debtlens-next.md
• npm run schema:generate