feat: silent fallback when no instrument — no oscillator default

_playNoteFluid no-instrument branch previously called
_playNoteFallback (sine wave oscillator). Changed to
silent return. User explicitly requested no default sound.

app/static/js/services/soundfontPlayer.js

@@ -393,12 +393,7 @@
                     _fluidModule._fluid_synth_program_change(_synthPtr, ch, usedProg);
                 } catch (e) {}
             } else {
-                // No instrument configured: use oscillator fallback directly.
-                // Do NOT call program_change(ch, 0) — FluidSynth would pick
-                // program 0 from whatever SoundFont is loaded (could be
-                // another track's SF), causing cross-track instrument leak.
-                var ctx = getCtx();
-                self._playNoteFallback(note, velocity, durationMs, startTime, undefined, null, channel, undefined);
+                // No instrument configured: silent — no FluidSynth, no oscillator.
                 return;
             }
             if (ch === undefined) ch = (usedBank === 128 ? 9 : 0);