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>

components/canvas/canvas-with-tools.tsx

@@ -3,6 +3,8 @@
 import { useEffect, useRef, useState } from 'react';
 import { useCanvasStore, useLayerStore, useToolStore } from '@/store';
 import { useHistoryStore } from '@/store/history-store';
+import { useSelectionStore } from '@/store/selection-store';
+import { drawMarchingAnts } from '@/lib/selection-utils';
 import { getContext, drawGrid, drawCheckerboard } from '@/lib/canvas-utils';
 import { DrawCommand } from '@/core/commands';
 import {
@@ -11,6 +13,10 @@ import {
   EraserTool,
   FillTool,
   EyedropperTool,
+  RectangularSelectionTool,
+  EllipticalSelectionTool,
+  LassoSelectionTool,
+  MagicWandTool,
   type BaseTool,
 } from '@/tools';
 import type { PointerState } from '@/types';
@@ -23,6 +29,11 @@ const tools: Record<string, BaseTool> = {
   eraser: new EraserTool(),
   fill: new FillTool(),
   eyedropper: new EyedropperTool(),
+  select: new RectangularSelectionTool(),
+  'rectangular-select': new RectangularSelectionTool(),
+  'elliptical-select': new EllipticalSelectionTool(),
+  'lasso-select': new LassoSelectionTool(),
+  'magic-wand': new MagicWandTool(),
 };
 
 export function CanvasWithTools() {
@@ -46,6 +57,8 @@ export function CanvasWithTools() {
   const { layers, getActiveLayer } = useLayerStore();
   const { activeTool, settings } = useToolStore();
   const { executeCommand } = useHistoryStore();
+  const { activeSelection, selectionType, isMarching } = useSelectionStore();
+  const [marchingOffset, setMarchingOffset] = useState(0);
 
   const [isPanning, setIsPanning] = useState(false);
   const [panStart, setPanStart] = useState({ x: 0, y: 0 });
@@ -113,18 +126,25 @@ export function CanvasWithTools() {
       drawGrid(ctx, gridSize, 'rgba(0, 0, 0, 0.15)');
     }
 
-    // Draw selection if active
-    if (selection.active) {
-      ctx.strokeStyle = '#0066ff';
-      ctx.lineWidth = 1 / zoom;
-      ctx.setLineDash([4 / zoom, 4 / zoom]);
-      ctx.strokeRect(selection.x, selection.y, selection.width, selection.height);
-      ctx.setLineDash([]);
+    // Draw selection if active (marching ants)
+    if (activeSelection && isMarching) {
+      drawMarchingAnts(ctx, activeSelection.mask, marchingOffset);
     }
 
     // Restore context state
     ctx.restore();
-  }, [layers, width, height, zoom, offsetX, offsetY, showGrid, gridSize, backgroundColor, selection, pointer]);
+  }, [layers, width, height, zoom, offsetX, offsetY, showGrid, gridSize, backgroundColor, selection, pointer, activeSelection, isMarching, marchingOffset]);
+
+  // Marching ants animation
+  useEffect(() => {
+    if (!activeSelection || !isMarching) return;
+
+    const interval = setInterval(() => {
+      setMarchingOffset((prev) => (prev + 1) % 8);
+    }, 50);
+
+    return () => clearInterval(interval);
+  }, [activeSelection, isMarching]);
 
   // Handle mouse wheel for zooming
   const handleWheel = (e: React.WheelEvent) => {
@@ -156,6 +176,31 @@ export function CanvasWithTools() {
       return;
     }
 
+    // Selection tools
+    const selectionTools = ['select', 'rectangular-select', 'elliptical-select', 'lasso-select', 'magic-wand'];
+    if (e.button === 0 && !e.shiftKey && selectionTools.includes(activeTool)) {
+      const activeLayer = getActiveLayer();
+      if (!activeLayer || !activeLayer.canvas) return;
+
+      const tool = tools[`${selectionType}-select`] || tools['select'];
+      const newPointer: PointerState = {
+        isDown: true,
+        x: canvasPos.x,
+        y: canvasPos.y,
+        prevX: canvasPos.x,
+        prevY: canvasPos.y,
+        pressure: e.pressure || 1,
+      };
+
+      setPointer(newPointer);
+
+      const ctx = activeLayer.canvas.getContext('2d');
+      if (ctx) {
+        tool.onPointerDown(newPointer, ctx, settings);
+      }
+      return;
+    }
+
     // Drawing tools
     if (e.button === 0 && !e.shiftKey && ['pencil', 'brush', 'eraser', 'fill', 'eyedropper'].includes(activeTool)) {
       const activeLayer = getActiveLayer();