Detection now runs off the UI isolate and covers every platform where
the answer can be trusted.
Availability was a plain platform check, so Linux always listed VLC, mpv
and Celluloid, macOS always listed VLC and IINA, and Windows always
listed VLC and PotPlayer whether or not any of them existed. Each player
now has a detector that asks exactly the question its launcher asks:
`sh -c 'command -v'` for PATH launches so the kernel performs the
executable check, NSWorkspace/Launch Services for `open -a`, `where.exe`
plus the concrete install paths for Windows VLC, and the registered URL
handler for PotPlayer and the iOS players.
Detection is asynchronous and memoised behind KnownPlayers.probe rather
than a Process.runSync in a static initialiser, which forked three
shells on the UI isolate during ExternalPlayerScreen.build. It is
prewarmed from startup, fails open when a probe throws, and keeps the
selected player listed when a detector misses it so a false negative
cannot leave the list with nothing selected.
iOS and tvOS gained LSApplicationQueriesSchemes entries for vlc and
infuse. Without them canOpenURL returns false for both schemes, so
_launchUrlScheme was already refusing to hand off to either player.
Android keeps the platform check: package visibility needs native
declarations, and a wrong answer there hides a working player.
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