v3.0.0

Migrated to prepare-package bundle mode with ESM, CJS, and IIFE output formats.
Refactored monolithic src/index.js into modular files under src/lib/.
Added window.Powertools assignment for CDN/IIFE usage and named Powertools export.
Added concurrency support to queue() function.
Rewrote README with clean grouped API documentation.
Removed .npmignore in favor of package.json files field.
Added dist/ to .gitignore. Deleted src/testground.js.
Updated CHANGELOG with v3.0.0 entry.