docs: record which download paths have actually been run

The download-to-disk story is three mechanisms — File System Access in
Chrome, a service worker streaming a response in Firefox and Safari, a
blob as the floor — and no test in this repository exercises any of them.
test_downloads.py pins their contracts by reading the source; whether a
browser really writes to disk needs a person with a large file.

One now has: Firefox, 180 MB, written to disk rather than assembled in
the tab. That is the path worth confirming, since it is the only one
Firefox has and it was written blind. It is also not the scale it exists
for, and the guide says which rows of that table are measured and which
are still only designed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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