v0.3.0 — playable: menus, browser, settings, calibration

First published build. BeatByte boots to a menu, generates its own demo song, and is genuinely playable on Medium–Expert. macOS (arm64/x86_64), Windows and Linux binaries attached — unzip and run beatbyte (macOS: right-click → Open on first launch, the binaries are unsigned). Full details in the CHANGELOG.