feat: add DOM-based prototype template system for full Inkscape editability

Implements a new templating approach that allows complete visual control
in Inkscape while maintaining 100% valid SVG.

New Features:
- DOM-based generator using DOMParser and cloneNode()
- Prototype elements (month-proto, lane-proto, item-proto) instead of string templates
- Full WYSIWYG editing in Inkscape - see exactly how timeline will look
- Auto-detection of template type (prototype vs template-v2)
- Text element mapping via IDs (e.g., id="item-title")
- SVG transforms for positioning instead of placeholder replacement

Implementation:
- generator-dom.js: New DOM-based generator with cloning logic
- engine.js: Auto-detect template type and use appropriate generator
- example-proto/: Complete working example with prototype template
- PROTOTYPE_TEMPLATES.md: Comprehensive guide for creating prototype templates

Benefits:
- No string placeholders ({{PLACEHOLDER}}) needed
- Native SVG editing workflow
- Better performance (DOM manipulation vs regex)
- Easier maintenance and styling
- Backward compatible (old template-v2 still works)

Template Structure:
- Prototypes with specific IDs visible in SVG (hidden after cloning)
- Container groups for generated content
- CSS classes for styling
- Text elements with IDs matching field names

All 56 tests still passing.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

example-proto/project.json

@@ -0,0 +1,21 @@
+{
+  "name": "Prototype Template Example",
+  "description": "Example using DOM-based prototype templates - fully editable in Inkscape!",
+  "dataSource": "sample.csv",
+  "stylesheet": "style.css",
+  "svgTemplate": "template-proto.svg",
+  "settings": {
+    "timelineMonths": 18,
+    "marginLeft": 220,
+    "marginTop": 140,
+    "monthWidth": 120,
+    "laneHeight": 80,
+    "laneGap": 16
+  },
+  "fieldMapping": {
+    "id": "ID",
+    "title": "Title",
+    "lane": "Lane",
+    "due": ["Due"]
+  }
+}