fix(mpv): stop loading the ytdl hook for media-server streams

Every URL the player opens is a media-server stream or a local file, so mpv's
bundled ytdl_hook has nothing to resolve. It still ran an on_load hook per
open and, whenever an open failed, spawned yt-dlp with the full stream URL in
its argv — access token included, readable through /proc on Linux. It also
added ~700ms to every failed open and buried the real "[stream] Failed to
open" line under three ytdl_hook errors.

mpv decides whether to load the builtin script inside mpv_initialize, so this
has to be an option set beforehand rather than a property set from Dart.
Verified against mpv 0.41: --ytdl=yes logs "Loading lua script
@ytdl_hook.lua", --ytdl=no never loads it.

Apple is deliberately excluded: the bundled libmpv is built without Lua, so
the option does not exist there and setting it would only print an mpv error
on every player init.

linux/runner/mpv/mpv_player.cc

@@ -346,6 +346,13 @@ bool MpvPlayer::Initialize() {
   mpv_set_option_string(mpv_, "input-vo-keyboard", "no");
   mpv_set_option_string(mpv_, "osc", "no");
   mpv_set_option_string(mpv_, "terminal", "no");
+  // Every URL Plezy opens is a media-server stream or a local file, never a
+  // site mpv's bundled ytdl_hook could resolve. Loading it costs an on_load
+  // hook per open and, on a failed open, spawns yt-dlp with the full stream
+  // URL — access token included — in its argv, where /proc exposes it. mpv
+  // gates loading the builtin script on this option at mpv_initialize time,
+  // so it has to be set here rather than from Dart.
+  mpv_set_option_string(mpv_, "ytdl", "no");
 
   // Default to warn-level logging
   mpv_request_log_messages(mpv_, "warn");