chore: bump version to v2026.4.308 and update build refs}
- Update package.json version from v2026.4.307 to v2026.4.308
- Update build version references from v2026.4.306 to v2026.4.307 across documentation files (README.md, INSTALL.md, AGENTS.md, change-log.md, change-break.md, todo.md, artifacts/php-7.2-ubuntu.md, artifacts/screenshots.md)