docs: update PLAN.md to reflect Phase 4 completion

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

PLAN.md

@@ -2,12 +2,13 @@
 
 ## Progress Overview
 
-**Current Status**: Phase 3 Complete ✓
+**Current Status**: Phase 4 Partial ✓
 
 ### Completed Phases
 - ✅ **Phase 1**: Project Setup & Core Infrastructure (95% complete)
 - ✅ **Phase 2**: Audio Engine Foundation (90% complete)
 - ✅ **Phase 3**: Waveform Visualization (95% complete)
+- ✅ **Phase 4**: Selection & Editing (70% complete - core editing features done)
 
 ### Working Features
 - ✅ Audio file upload with drag-and-drop
@@ -15,20 +16,26 @@
 - ✅ Playback controls (play, pause, stop, seek)
 - ✅ Volume control with mute
 - ✅ Timeline scrubbing
-- ✅ **Drag-to-scrub audio** (NEW!)
+- ✅ Drag-to-scrub audio
-- ✅ **Horizontal zoom (1x-20x)** (NEW!)
+- ✅ Horizontal zoom (1x-20x)
-- ✅ **Vertical amplitude zoom** (NEW!)
+- ✅ Vertical amplitude zoom
-- ✅ **Scroll through zoomed waveform** (NEW!)
+- ✅ Scroll through zoomed waveform
-- ✅ **Grid lines every second** (NEW!)
+- ✅ Grid lines every second
-- ✅ **Viewport culling for performance** (NEW!)
+- ✅ Viewport culling for performance
+- ✅ **Region selection with Shift+drag** (NEW!)
+- ✅ **Visual selection feedback** (NEW!)
+- ✅ **Cut/Copy/Paste operations** (NEW!)
+- ✅ **Delete and Trim operations** (NEW!)
+- ✅ **Keyboard shortcuts (Ctrl+A/X/C/V, Delete, Escape)** (NEW!)
+- ✅ **Clipboard management** (NEW!)
 - ✅ Dark/light theme support
 - ✅ Toast notifications
 - ✅ File metadata display
 
 ### Next Steps
-- Selection & region editing (Phase 4)
 - Undo/redo system (Phase 5)
 - Audio effects (Phase 6)
+- Multi-track editing (Phase 7)
 
 ---
 
@@ -312,11 +319,11 @@ audio-ui/
 ### Phase 4: Selection & Editing
 
 #### 4.1 Region Selection
-- [ ] Click-and-drag selection
+- [x] Click-and-drag selection (Shift+drag)
 - [ ] Keyboard-based selection (Shift+Arrow)
-- [ ] Select all (Ctrl+A)
+- [x] Select all (Ctrl+A)
-- [ ] Clear selection (Escape)
+- [x] Clear selection (Escape)
-- [ ] Selection visual feedback
+- [x] Selection visual feedback
 
 #### 4.2 Region Markers
 - [ ] Add/Remove region markers
@@ -326,16 +333,16 @@ audio-ui/
 - [ ] Loop region
 
 #### 4.3 Basic Edit Operations
-- [ ] Cut (Ctrl+X)
+- [x] Cut (Ctrl+X)
-- [ ] Copy (Ctrl+C)
+- [x] Copy (Ctrl+C)
-- [ ] Paste (Ctrl+V)
+- [x] Paste (Ctrl+V)
-- [ ] Delete (Delete key)
+- [x] Delete (Delete key)
-- [ ] Trim to selection
+- [x] Trim to selection
 - [ ] Split at cursor
 - [ ] Duplicate selection
 
 #### 4.4 Clipboard Management
-- [ ] Internal clipboard for audio data
+- [x] Internal clipboard for audio data
 - [ ] Multiple clipboard slots (optional)
 - [ ] Clipboard preview