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);

wiki.md

@@ -539,10 +539,10 @@
 - **Ghi chú/Test (nếu có):** `npm run build` pass. Ctrl+Click+Drag section/MIDI item → copy đến vị trí mới, item không bị selected.
 ---
 
-### [2026-07-28 08:17] Task: Kế thừa instrument từ MAIN track vào section sub-tracks
-- **Tóm tắt thay đổi:** Fix fallback branch `handleEditSectionInTab` — khi section chưa có tracks riêng, clone tracks kế thừa `instrumentId`, `instrumentProgram`, `instrumentName` từ parent MAIN track thay vì null/undefined. Trước đây instrument bị xoá (null/undefined) → section không có instrument nào.
+### [2026-07-28 08:27] Task: KHÔNG auto-load instrument MAIN vào SECTION-TAB track
+- **Tóm tắt thay đổi:** Revert kế thừa instrument từ parent MAIN track. Section tracks giữ `instrumentId: null, instrumentProgram: undefined, instrumentName: null, synth_engine: undefined`. Trước đây kế thừa instrument (f2ac70e) → SECTION-TAB auto-load program 0 của SoundFont từ MAIN. User tự chọn instrument trong SECTION-TAB qua Synth button (đã hỗ trợ qua `updateActiveTracks` routing).
 - **Các file ảnh hưởng:** `app/static/js/app.jsx`, `app/static/js/app.precompiled.js`
-- **Ghi chú/Test (nếu có):** `npm run build` pass. Load instrument trên MAIN track → mở section tab → MIDI items kế thừa instrument từ parent track.
+- **Ghi chú/Test (nếu có):** `npm run build` pass. Load instrument MAIN → mở section tab → section tracks KHÔNG có instrument.
 ---
 
 ### [2026-07-28 08:12] Task: Sync instrument load methods từ MAIN vào SECTION-TAB PIANO_ROLL