security: fix all npm vulnerabilities

Resolved 3 vulnerabilities:
- serialize-javascript RCE (HIGH, CVSS 8.1) - GHSA-5c6j-r48x-rmvq
- serialize-javascript DoS (MODERATE, CVSS 5.9) - GHSA-qj8w-gfj5-8c6v
- diff DoS (LOW) - GHSA-73rr-hh4g-fpgx

Changes:
- Downgrade mocha from 11.7.5 to 11.3.0
- Add npm overrides for diff@^5.2.0 (safe version)
- Add npm overrides for serialize-javascript@^7.0.5 (safe version)

Verification:
✅ npm audit: 0 vulnerabilities
✅ All 10 tests passing
✅ Code coverage: 98.96% (maintained)
✅ No breaking changes

Author: jkyberneees