Evaluate optional Rust helper for doctor and launcher only

[xodapi]

Why

A single Windows executable could make diagnostics and startup easier for non-developer colleagues, but rewriting the working Node proxy would add risk.

Scope

Evaluate a small Rust helper that keeps the current Node proxy and HTTP endpoints:

• opencode-proxy-doctor.exe;
• launcher/tray/startup helper;
• reader for existing usage.jsonl and future durable state snapshots;
• JSON output compatible with existing scripts.

Acceptance criteria

• Decision doc explains what stays in Node and what belongs in Rust.
• Prototype, if built, does not replace the proxy runtime.
• No data migration required for existing users.