v0.4.1

docs: rewrite README as a product landing page

Reframe the repo's front page from "developer resources" to a download-first
product page: hero tagline, per-platform download table (latest-release direct
links), Free vs Pro feature breakdown, DAW compatibility, and the developer/AI
resources folded in as a section. Claims mirror the donutsdelivery.online/arbit
copy. No content files moved; only README.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>