Network.getResponseBody returns the whole body in one base64 blob and the
cache drops anything over 2 MiB, so large downloads (images, PDFs,
archives) either failed or forced the long-running server to hold the
full body in memory (issue #360).
Add Fetch.takeResponseBodyAsStream + IO.read + IO.close. The body is moved
out of the page cache into a stream handle and delivered in bounded chunks
via IO.read, then freed on IO.close, so it is held once and released after
the download. Binary bodies are byte-exact (stored base64). To stream a
large body, raise OBSCURA_NETWORK_BODY_BUFFER_BYTES so it is cached.
This is entirely opt-in: nothing runs unless a client calls the new
methods, so the default fetch/render/scrape path is unchanged (verified:
obstacle course 33/33, obscura-vs-Chrome head-to-head within noise of the
pre-change baseline).
Fixes#360.
SGavrl
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