chore: bump version to v2026.4.273 and rewrite README}
- Bump package.json version from 2026.4.272 to 2026.4.273 - Update build references from v2026.4.271 to v2026.4.272 across all doc files - Rewrite README.md for clarity and conciseness: consolidate feature list, streamline development/build instructions, reorganize sections, and reduce overall length by ~40 lines - Affected files: AGENTS.md, INSTALL.md, README.md, artifacts/php-7.2-ubuntu.md, artifacts/screenshots.md, change-break.md, change-log.md, docs/migration-all-packages.md, package.json, todo.md