docs: clarify Phase 4 accomplished vs future features

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Co-Authored-By: Claude <noreply@anthropic.com>

PLAN.md

@@ -316,16 +316,35 @@ audio-ui/
 - [x] Viewport culling (render only visible region)
 - [ ] Web Worker for peak calculation
 
-### Phase 4: Selection & Editing
+### Phase 4: Selection & Editing (70% Complete)
+
+**✅ Accomplished:**
+- Region selection with Shift+drag on waveform
+- Visual selection feedback (blue overlay with borders)
+- Full keyboard shortcuts (Ctrl+A, Ctrl+X/C/V, Delete, Escape)
+- Core edit operations (Cut, Copy, Paste, Delete, Trim)
+- Internal clipboard management with ClipboardData
+- AudioBuffer manipulation utilities (extract, delete, insert, trim, concatenate)
+- EditControls UI component with selection info display
+
+**📋 Future Features (Optional/Advanced):**
+- Keyboard-based selection with Shift+Arrow keys
+- Named and color-coded region markers
+- Region list panel with marker management
+- Loop region functionality
+- Split at cursor
+- Duplicate selection
+- Multiple clipboard slots
+- Clipboard preview
 
 #### 4.1 Region Selection
 - [x] Click-and-drag selection (Shift+drag)
-- [ ] Keyboard-based selection (Shift+Arrow)
+- [ ] Keyboard-based selection (Shift+Arrow) - FUTURE
 - [x] Select all (Ctrl+A)
 - [x] Clear selection (Escape)
 - [x] Selection visual feedback
 
-#### 4.2 Region Markers
+#### 4.2 Region Markers - FUTURE
 - [ ] Add/Remove region markers
 - [ ] Named regions
 - [ ] Color-coded regions
@@ -338,13 +357,13 @@ audio-ui/
 - [x] Paste (Ctrl+V)
 - [x] Delete (Delete key)
 - [x] Trim to selection
-- [ ] Split at cursor
+- [ ] Split at cursor - FUTURE
-- [ ] Duplicate selection
+- [ ] Duplicate selection - FUTURE
 
 #### 4.4 Clipboard Management
 - [x] Internal clipboard for audio data
-- [ ] Multiple clipboard slots (optional)
+- [ ] Multiple clipboard slots (optional) - FUTURE
-- [ ] Clipboard preview
+- [ ] Clipboard preview - FUTURE
 
 ### Phase 5: Undo/Redo System