Refactor AR.js-core to ECS-only: remove legacy code and update examples/docs

[kalwalt]

Overview

Transition AR.js-core to an ECS-only codebase as the foundation for AR.js-next.

Goals

• Remove all legacy/non-ECS code: old classes, adapters, new-api, arjs-source/arjs-profile
• Export only ECS core, systems, plugin manager, recommended plugins from src/index.js
• Update all examples and docs to showcase ECS/plugin workflows
• Add migration note to the README (legacy API split/moved)
• Prepare for future external plugins and renderer packages (e.g., arjs-plugin-threejs)

Checklist

• Remove src/arjs-source, src/arjs-profile, src/new-api/*, any legacy adapters
• Remove legacy exports from src/index.js
• Migrate all examples to ECS workflow and bundled library imports (no legacy paths)
• Update README with migration note, ECS intro, usage sample, plugin info
• CI and build scripts unchanged (only ECS and plugin code present)
• Bump version (e.g. 0.3.0 or 1.0.0-next depending on future plans)
• Consider (optionally) publishing legacy API as a separate arjs-legacy-adapter package

Notes

• First step before external plugins: after this, Three.js integration moves to arjs-plugin-threejs, not core.
• For now, keep all code in JavaScript (not Typescript) for consistency.
• This work will clarify AR.js-core as the modular root for all next-generation AR.js packages.