Devlog — Countdown fix + WASM sync

Fixed countdown being on by default when converting Etterna to osu. Added
Countdown: 0 to the [General] section in the converter, the dummy diff
generator, and the TypeScript equivalent.

Discovered the web version was using a stale pre-built WASM binary that
didn’t have any of the Rust fixes. Created wasm-core/ — a new crate that
builds WASM from the same Rust source via #[path] references. No code
duplication.

Added npm run build:wasm for local rebuilds and a CI job that auto-rebuilds
WASM on push to main so it never goes stale again