feat: add custom placeholder mapping for non-standard templates

Added optional placeholderMapping to project.json for templates that
don't follow the standard ITEM_{PROPERTY} convention.

Features:
- Override default placeholder names per field
- Useful for legacy templates or migration from other tools
- Fallback to ITEM_{PROPERTY} convention when not specified

Example usage:
{
  "fieldMapping": {
    "id": "ID",
    "title": "Title"
  },
  "placeholderMapping": {
    "id": "TASK_ID",     // Use {{TASK_ID}} instead of {{ITEM_ID}}
    "title": "TASK_NAME" // Use {{TASK_NAME}} instead of {{ITEM_TITLE}}
  }
}

Changes:
- generator.js: Check for cfg.placeholderMapping before using default
- Added 2 new tests for custom mapping behavior
- Updated TEMPLATE_V2_GUIDE.md with documentation and examples

All 58 tests passing.

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

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

test/generator.test.js

@@ -278,5 +278,52 @@ describe('Timeline Generator', () => {
       expect(viewBoxMatch[1]).toBe(widthMatch[1])
       expect(viewBoxMatch[2]).toBe(heightMatch[1])
     })
+
+    it('should support custom placeholder mapping', () => {
+      const customConfig = {
+        ...config,
+        placeholderMapping: {
+          id: 'TASK_ID',      // Map item.id to {{TASK_ID}} instead of {{ITEM_ID}}
+          title: 'TASK_NAME'  // Map item.title to {{TASK_NAME}} instead of {{ITEM_TITLE}}
+        }
+      }
+
+      // Create template with custom placeholders
+      const customTemplate = `<svg xmlns="http://www.w3.org/2000/svg">
+        <defs>
+          <g id="month-template" style="display:none">
+            <text x="{{MONTH_TEXT_X}}" y="{{MONTH_LABEL_Y}}">{{MONTH_LABEL}}</text>
+          </g>
+          <g id="lane-template" style="display:none">
+            <text>{{LANE_NAME}}</text>
+          </g>
+          <g id="item-template" style="display:none">
+            <text x="{{TEXT_X}}" y="{{TEXT_Y}}">{{TASK_ID}}: {{TASK_NAME}}</text>
+          </g>
+        </defs>
+        {{MONTHS}}
+        {{LANES}}
+      </svg>`
+
+      const result = timelineGenerator.generate(items, customConfig, customTemplate)
+
+      // Should use custom placeholder names
+      expect(result).toContain('T-1: First Task')  // TASK_ID: TASK_NAME
+      expect(result).toContain('T-2: Second Task')
+
+      // Should not contain default placeholder syntax
+      expect(result).not.toContain('{{ITEM_ID}}')
+      expect(result).not.toContain('{{ITEM_TITLE}}')
+    })
+
+    it('should use default convention when placeholderMapping is not provided', () => {
+      const template = createSampleTemplate()
+      const result = timelineGenerator.generate(items, config, template)
+
+      // Should use default ITEM_* convention
+      expect(result).toContain('T-1 First Task')  // Default template has space, not colon
+      expect(result).not.toContain('{{ITEM_ID}}')
+      expect(result).not.toContain('{{ITEM_TITLE}}')
+    })
   })
 })