docs: add memory leak flag investigation documentation

Document findings from investigating FF_LSDV_4620_3_ML and FF_DEV_3391
feature flags:

- FF_LSDV_4620_3_ML is explicitly disabled due to LSE/LSO React build
  differences, causing memory leaks in V17 destroy path
- FF_DEV_3391 controls per-annotation config trees for View All mode
- Includes mitigation strategies that work without enabling flags
- Provides testing recommendations for re-enabling the flag

Author: bmartel

web/libs/editor/src/MEMORY_INVESTIGATION.md

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+# Memory Leak Flag Investigation
+
+## Overview
+
+This document summarizes the investigation into memory-related feature flags in the Label Studio Editor:
+- `FF_LSDV_4620_3_ML` - Memory leak fixes
+- `FF_DEV_3391` - Per-annotation config trees
+
+## FF_LSDV_4620_3_ML (Memory Leak Fixes)
+
+### Current Status: **DISABLED by default**
+
+The flag is explicitly overridden to `false` in `utils/feature-flags.ts`:
+
+```typescript
+const override: Record<string, boolean> = {
+  fflag_fix_front_lsdv_4620_memory_leaks_100723_short: false,
+};
+```
+
+With this comment:
+> TODO: remove the override + if statement once LSE and LSO start building react the same way
+
+### What the Flag Controls
+
+In `LabelStudio.tsx` (V17 destroy path), when **enabled**:
+
+1. `clearRenderedApp()` - Properly unmounts React before MST destruction
+2. `this.store.selfDestroy()` - Walks MST tree and destroys children in order
+3. Nulls out `this.store`, `this.destroy`, removes from `LabelStudio.instances`
+
+When **disabled** (current default):
+
+1. ❌ React is NOT unmounted before MST destruction
+2. ❌ MST children are not destroyed in order
+3. ❌ References are not nulled out → **Memory leaks**
+
+### Why It's Disabled
+
+The comment indicates there's an incompatibility between how:
+- Label Studio Enterprise (LSE)
+- Label Studio Open Source (LSO)
+
+...build React. The specific issue is not documented, but likely relates to:
+- Different React versions (17 vs 18)
+- Different bundling configurations
+- Race conditions in the unmount/destroy sequence
+
+### Recommended Actions
+
+1. **Test enabling the flag** in LSO-only environment to document exact failures
+2. **Identify the React build difference** between LSE and LSO
+3. **Fix root cause** rather than keeping the flag disabled
+4. Alternatively, implement flag-independent cleanup for critical paths:
+   - Always clear `window.Htx` (prevents store retention)
+   - Always null out `this.store` after destruction
+   - Add `isAlive` guards to all async operations
+
+## FF_DEV_3391 (Per-Annotation Config Trees)
+
+### Status: Feature flag for "View All" mode
+
+This flag enables separate config trees per annotation, required for interactive annotations in View All mode.
+
+### Impact on Memory
+
+When enabled:
+- Each annotation gets its own config tree copy
+- Memory scales linearly with annotation count
+- Helps prevent cross-annotation state pollution
+
+When disabled:
+- Single shared config tree
+- Lower memory usage
+- View All mode is view-only
+
+### Location
+
+Used in `stores/Annotation/Annotation.js` to conditionally create per-annotation root trees.
+
+## Memory Leak Mitigation Without Flags
+
+Even with flags disabled, these patterns help reduce leaks:
+
+### 1. Clear Global References
+
+```javascript
+// Always do this in destroy(), regardless of flags
+window.Htx = null;
+```
+
+### 2. Guard Async Operations
+
+```javascript
+setTimeout(() => {
+  if (!isAlive(self)) return;  // Guard against destroyed store
+  // ... rest of logic
+});
+```
+
+### 3. Dispose Subscriptions
+
+```javascript
+let disposer = onSnapshot(self.areas, callback);
+// In beforeDestroy:
+disposer?.();
+```
+
+### 4. Use Safe References
+
+Where possible, use `types.safeReference()` instead of `types.reference()` to avoid warnings during destruction.
+
+## Testing Recommendations
+
+1. Enable `FF_LSDV_4620_3_ML` in test environment
+2. Navigate in/out of labeling interface
+3. Monitor:
+   - Memory heap snapshots
+   - Console warnings
+   - JS errors
+4. Document exact failure conditions
+5. Fix root causes before re-enabling