feat(phase-8): implement comprehensive selection system with marching ants

This commit completes Phase 8 of the paint-ui implementation, adding a full
selection system with multiple selection tools, operations, and visual feedback.

**New Files:**
- types/selection.ts: Selection types, masks, and state interfaces
- lib/selection-utils.ts: Selection mask generation and manipulation algorithms
- lib/selection-operations.ts: Copy/cut/paste/delete/fill/stroke operations
- store/selection-store.ts: Selection state management with Zustand
- core/commands/selection-command.ts: Undo/redo commands for selections
- tools/rectangular-selection-tool.ts: Rectangular marquee selection
- tools/elliptical-selection-tool.ts: Elliptical marquee selection
- tools/lasso-selection-tool.ts: Free-form polygon selection
- tools/magic-wand-tool.ts: Color-based flood-fill selection
- components/selection/selection-panel.tsx: Complete selection UI panel
- components/selection/index.ts: Selection components barrel export

**Updated Files:**
- components/canvas/canvas-with-tools.tsx: Added selection tools integration and marching ants animation
- components/editor/editor-layout.tsx: Integrated SelectionPanel into layout
- store/index.ts: Added selection-store export
- store/canvas-store.ts: Renamed Selection to CanvasSelection to avoid conflicts
- tools/index.ts: Added selection tool exports
- types/index.ts: Added selection types export
- types/canvas.ts: Renamed Selection interface to CanvasSelection

**Selection Tools:**

**Marquee Tools:**
- ✨ Rectangular Selection: Click-drag rectangular regions
- ✨ Elliptical Selection: Click-drag elliptical regions

**Free-form Tools:**
- ✨ Lasso Selection: Draw free-form polygon selections
- ✨ Magic Wand: Color-based flood-fill selection with tolerance

**Selection Modes:**
- 🔷 New: Replace existing selection
- ➕ Add: Add to existing selection
- ➖ Subtract: Remove from existing selection
- ⚡ Intersect: Keep only overlapping areas

**Selection Operations:**
- 📋 Copy/Cut/Paste: Standard clipboard operations with selection mask
- 🗑️ Delete: Remove selected pixels
- 🎨 Fill Selection: Fill with current color
- 🖌️ Stroke Selection: Outline selection with current color
- 🔄 Invert Selection: Invert selected/unselected pixels
- ❌ Clear Selection: Deselect all

**Technical Features:**
- Marching ants animation (animated dashed outline at 50ms interval)
- Selection masks using Uint8Array (0-255 values for anti-aliasing)
- Feathering support (0-250px gaussian blur on selection edges)
- Tolerance control for magic wand (0-255 color difference threshold)
- Scanline polygon fill algorithm for lasso tool
- Flood-fill with Set-based visited tracking for magic wand
- Selection bounds calculation for optimized operations
- Keyboard shortcuts (Ctrl+C, Ctrl+X, Ctrl+V, Ctrl+D, Ctrl+Shift+I)
- Undo/redo integration via selection commands
- Non-destructive operations with proper history tracking

**Algorithm Implementations:**
- Rectangular mask: Simple bounds-based pixel marking
- Elliptical mask: Distance formula from ellipse center
- Lasso mask: Scanline polygon fill with edge intersection
- Magic wand: BFS flood-fill with color tolerance matching
- Mask combination: Per-pixel operations (max, subtract, AND)
- Feathering: Separable box blur (horizontal + vertical passes)
- Mask inversion: Per-pixel NOT operation with bounds recalculation

**UI/UX Features:**
- 264px wide selection panel with all tools and operations
- Tool selection with visual feedback (highlighted active tool)
- Selection mode toggles (new/add/subtract/intersect)
- Feather and tolerance sliders with live value display
- Disabled state when no selection exists
- Keyboard shortcut hints next to operations
- Visual marching ants animation (animated dashes)

Build verified: ✓ Compiled successfully in 1302ms

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

types/canvas.ts

@@ -25,9 +25,9 @@ export interface CanvasState {
 }
 
 /**
- * Selection interface for selected regions
+ * Canvas selection interface for selected regions (deprecated - use Selection from selection.ts)
  */
-export interface Selection {
+export interface CanvasSelection {
   /** Is there an active selection */
   active: boolean;
   /** Selection bounds */

types/index.ts

@@ -3,3 +3,4 @@ export * from './layer';
 export * from './tool';
 export * from './history';
 export * from './filter';
+export * from './selection';

types/selection.ts

@@ -0,0 +1,47 @@
+export type SelectionType = 'rectangular' | 'elliptical' | 'lasso' | 'magic-wand';
+
+export type SelectionMode = 'new' | 'add' | 'subtract' | 'intersect';
+
+export interface SelectionBounds {
+  x: number;
+  y: number;
+  width: number;
+  height: number;
+}
+
+/**
+ * Selection mask is a 2D boolean array representing selected pixels
+ * true = selected, false = not selected
+ */
+export interface SelectionMask {
+  width: number;
+  height: number;
+  data: Uint8Array; // 1 byte per pixel (0 = not selected, 255 = selected)
+  bounds: SelectionBounds;
+}
+
+export interface Selection {
+  id: string;
+  layerId: string;
+  mask: SelectionMask;
+  inverted: boolean;
+  feather: number; // Feather radius in pixels
+  createdAt: number;
+}
+
+export interface SelectionState {
+  activeSelection: Selection | null;
+  selectionType: SelectionType;
+  selectionMode: SelectionMode;
+  feather: number;
+  tolerance: number; // For magic wand (0-255)
+  isMarching: boolean; // Marching ants animation
+}
+
+/**
+ * Point for lasso selection
+ */
+export interface LassoPoint {
+  x: number;
+  y: number;
+}