Sounds play on flutter_soloud instead of three mpv players

SoundEngine (platform/audio) opens the device once (2 s after start,
or on the first sound), loads each synthesized sound into memory (no
temp files), plays overlapping voices and is disposed on window close,
before leaving the server. The SfxService interface and its callers
are unchanged; loudness matches the old mpv path (v^6). WAV only, so
no codec libraries are bundled and no build packages are needed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DEVELOPMENT.md

@@ -6,7 +6,11 @@ Layout and rules: [ARCHITECTURE.md](ARCHITECTURE.md).
 
 Needs the Flutter SDK (beta channel, Dart 3.13+) and, for the Linux desktop
 build, the usual GTK toolchain (clang, cmake, ninja, pkg-config, libgtk-3-dev).
-mpv libraries come from `media_kit_libs_video`. `fox_utils` (drag & drop,
+mpv libraries come from `media_kit_libs_video`. `flutter_soloud` (the app's
+sounds) compiles SoLoud and miniaudio from source in its build hook with
+that C++ toolchain (MSVC on Windows); it needs no audio dev headers (miniaudio
+loads ALSA / PulseAudio / JACK at run time) and, with `no_xiph_libs` set in
+the desktop pubspec, downloads nothing. `fox_utils` (drag & drop,
 clipboard files and images; native Linux/Windows plugin) is the owner's
 shared package, a git dependency pinned to a tag of
 `git.senkochan.moe/Foxyz/fox_utils`: bump it by tagging there and changing
@@ -67,6 +71,9 @@ and `e2e_check` (simulated viewers against a live server).
   media_kit "S/W rendering" because they have no display; that says nothing
   about the app. Run only tests related to
   what you changed.
+- `sound_device_e2e` opens the real audio device (flutter_soloud) and plays
+  every sound of every preset quietly (`WT_SFX_VOLUME`, default 0.1);
+  `sound_engine_test` covers the engine with a fake backend.
 - CI (`.gitea/workflows/ci.yml`): analyze + test for core and server, plus
   `dart compile exe` of the dedicated server (fails if Flutter creeps into
   `packages/server`). Desktop tests are not in CI.
@@ -184,7 +191,7 @@ slow to come up shows mpv's error with "no HTTP error logged".
 - media_kit has no public `Player.setProperty`; the code reaches mpv through a
   dynamic call on `player.platform`. Cache size goes through
   `PlayerConfiguration(bufferSize:)`.
-- Hot restart with any media_kit `Player` alive (session, thumbnails, sfx)
+- Hot restart with any media_kit `Player` alive (session, thumbnails)
   used to SIGSEGV on mpv's `core` thread: each mpv handle's wakeup callback
   is a `NativeCallable` of the old isolate, freed by the restart, and media_kit
   then sends the leftover handles `quit`, which fires it. `fox_utils`' Linux