chore: bump version to v2026.4.271 and update build references}
- Update package.json version from v2026.4.270 to v2026.4.271
- Update build version references from v2026.4.268 to v2026.4.270 across documentation and artifact files (README.md, INSTALL.md, AGENTS.md, change-log.md, change-break.md, todo.md, artifacts/php-7.2-ubuntu.md, artifacts/screenshots.md, docs/migration-all-packages.md)