[fabox-os]

Choosing a .sf2 (or opening a project with one) used to read and parse the whole file (~150 MB for GM) on the UI thread, holding it twice in memory.

• SoundFontBank::load_reader: parse straight from a buffered reader (no second copy).
• soundfonts::request/progress: background thread, progress from bytes read, time left from the pace; failures remembered until forget.
• engine_sync: a track waits (keeps its synth patch) while its file loads; the window keeps repainting.
• Inspector: progress bar "Loading… 42% · about 3 s left" in the SoundFont row; 4 new texts in all 16 languages.
• Tests: progress estimate and text, failed file reported from the background thread. Linux fmt, clippy -D warnings, workspace tests and wasm32 engine check pass.
• Not measured: a real GM SoundFont (none on this machine), so the speed-up is by reasoning (no UI-thread block, one copy), not a benchmark. Not tested on Windows/macOS.

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