docs: what this session cost to find

Five lessons, and the first is the expensive one: a test that models a fix
agrees with it by construction. The buffer-ceiling test passed against a player
that still hung, because the model and the fix had the same author and the same
misunderstanding.

Also: `no-cache` only binds a browser that asks; redeploying during someone
else's test kills their session and truncates the log holding the reproduction;
`updateend` fires for `remove()`; and flow control on a media stream is a
window, not a debt.

USERGUIDE section 7 rewritten — it still described 24 segments in flight and
two transcode slots, and said "transcode" where ffmpeg does a `-c copy` remux,
which is exactly why a slot costs little and why 500 MB really does go on the
wire.

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