Reads bun-manifest.json and resolves source paths to fingerprinted URLs.

Runs as a supervised GenServer that owns a public ETS table for lock-free reads. On every lookup the manifest file's mtime is checked and the cache is reloaded if it changed. That keeps dev builds fresh without polling.

Example

iex> BunBundle.Manifest.lookup("js/app.js")
%BunBundle.Manifest.Entry{url: "js/app-abc123.js", sri: ["sha384-..."]}