Add 'wl create --stream <url>' to torrentify a remote origin

[iksnerd]

The breadth track from the dataset strategy (follow-up to #9): register a remote artifact — a Hugging Face / Kaggle / any http(s) URL — without downloading it to disk. The body is streamed once to compute the hybrid v1+v2 hashes, and the origin URL is carried as a BEP 19 web seed so clients fetch from it. This is what lets us register many artifacts cheaply (no storage per item, no persistent seeder).

Changes

• torrent: refactored hashing so the core loop runs from an io.Reader + known size (hashReaderHybrid); split out populateSingleFileInfo and finalize so the on-disk and streaming paths share assembly. New CreateStream(opts, r, size, name). No behavior change to Create — golden vectors still pass.
• wl create: --stream <url> (mutually exclusive with a path arg). Fetches the URL, requires a known Content-Length, streams to CreateStream, auto-adds the origin as a web seed. Name derived from the URL path or --name.

Verification

• TestCreateStream asserts the streamed hashes equal the Transmission-verified golden vectors (proving streaming-hash == file-hash) and that the origin is recorded as a web seed; rejects bad size/name.
• End-to-end against a local HTTP server: wl create --stream http://…/data.bin produced v1 cc1614c7… / v2 db4ca38f… with no local copy, and transmission-show lists the web seed.
• go build / go vet / gofmt clean; full go test ./... green.

Constraints / follow-up

• Requires the origin to report Content-Length (chunked responses are rejected rather than buffered).
• Single-file only for now (one URL → one file). Multi-file remote sets are a later follow-up.
• wl get consumer-side web-seed download remains the other tracked follow-up.

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