v0.3.0: optional Laya and a user manual

Laya is now optional, so you only download what you use:

Install	What you get	Size
pip install "luar[all]"	web interface + Laya engine (recommended)	~1.5 GB on first use
pip install "luar[ui]"	web interface only, for LM Studio users	~200 MB
pip install "luar[laya]"	adds Laya later

Without Laya, the CLI and the web interface default to LM Studio and explain how to add Laya.

The README is now a step-by-step user manual (English, português) with screenshots. It covers:

• writing good questions
• what each result column means
• checking accuracy with expected_ columns
• choosing an engine
• troubleshooting

Developer notes moved to CONTRIBUTING.md.

Fixes:

• Download sizes: the README now gives the real sizes (Laya model 650–800 MB per variant, plus PyTorch).
• Linux: new note on installing CPU-only PyTorch.

Upgrading from 0.2.0: pip install "luar[ui]" no longer installs Laya. Use luar[all] to keep both engines.