feat(plugin): serve HttpDownloader as a proxy stub

A downloader carries its own options, TLS defaults and request backends, and
what it shares with the graph is the IO it collects authentication into and
the config it reads. Plugin code writing `new HttpDownloader($io, $config)`
therefore allocates a Rust-side entity of its own rather than a second
downloader the graph knows nothing about, and the guard that shadowed the
class in the worker is gone.

The request surface is not served yet: get() and copy() have no wire
representation for the Response they return, and the async surface resolves
its promises with one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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