revert: section tracks inherit parent instrument, auto-loads SF prog0

Previous fix f2ac70e inherited instrumentId/program/name
from parent MAIN track. This causes SECTION-TAB to auto-load
program 0 of whatever SoundFont is loaded on MAIN, even
though user didn't select any instrument for the section.
Section tracks now keep null instrument + synth_engine
undefined. User can still load instrument manually via
Synth button (routed by updateActiveTracks fix).

app/static/js/app.jsx

@@ -8514,9 +8514,10 @@ const App = () => {
       markers: [],
       isArmed: false,
       monitoringEnabled: true,
-      instrumentId: t.instrumentId || null,
-      instrumentProgram: t.instrumentProgram,
-      instrumentName: t.instrumentName || null
+      instrumentId: null,
+      instrumentProgram: undefined,
+      instrumentName: null,
+      synth_engine: undefined
     }));
     setSessionTabs(prev => [...prev, { id: tabId, name: tabName, sectionId: sectionId, tracks: clonedTracks, color: section.color || null }]);
     setActiveTab(tabId);