:download_disposition — flips the explicit :disposition option of
Phoenix.Controller.send_download/3 between :attachment and :inline. A surviving
mutant means no test pins whether the browser is told to save the file or display
it — the type in the content-disposition header.
send_download(conn, {:file, path}, disposition: :attachment) # → :inline
conn |> send_download({:binary, csv}, filename: "r.csv", disposition: :inline)
# ↳ :attachmentOnly an explicit literal atom is flipped: a send_download that relies on Phoenix's
default (:attachment), or passes a variable, is left alone — as is send_download/2,
which has no options at all. Matches the call written directly
(Phoenix.Controller.send_download(conn, ...)), aliased, or bare-imported
(send_download(conn, ...), the form use MyAppWeb, :controller produces).
The swap substitutes exactly the disposition atom, so Mutare.Transform.Overlap prunes
the built-in :atom family's :mutare leaf at that range — which would only crash
(Phoenix rejects any disposition other than :attachment / :inline), an uninformative
kill.