fix(plugins): step VST3 stepped params by 1/stepCount instead of 1 (#49)

[Horuse]

What does this PR do?

VST3 params with stepCount > 0 showed only 0/1: VST3 ranges are normalised 0..1, but the slider used step = 1 for stepped params. stepped: bool is replaced by step: f64 (VST3: 1 / stepCount, CLAP/AU: 1, continuous: 0).

Why is this the right approach?

Only the backend knows the step count. Converting VST3 params to plain units instead would touch every write path.

Checklist

• Diff is limited to the change — no unrelated edits
• bun run check passes
• cargo check --manifest-path src-tauri/Cargo.toml passes
• bun run format leaves the tree clean
• Generated TS types are committed with the Rust change (if any) — n/a
• No new dependency without a reason in the PR description
• I read the RT audio path section of docs/CONCEPT.md and confirmed this change adds no allocations, locks, or syscalls to cpal / SCK callbacks or DspWorker::run

Platform coverage

• Developed on: macOS 14
• Tested on: macOS 14, Fedora 44, Windows 11
• What I did to test: Tested Fabfilter Pro-MB and other vst3 plugins

au_host.rs (macOS only): field rename, behaviour unchanged.

Related

• Fixes [bug] Parameters of certain VST3 plugins are listed as binary rather than floating point in plugin node #49