feat(baoyu-cover-image): add font dimension with 4 styles

Add 6th dimension for typography control:
- clean: modern geometric sans-serif
- handwritten: warm hand-lettered style
- serif: classic elegant typography
- display: bold decorative headlines

Includes auto-selection rules, compatibility matrix, and warm-flat preset.

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

skills/baoyu-cover-image/references/compatibility.md

@@ -48,3 +48,12 @@
 | metaphor | ✓✓ | ✓✓ | ✓ |
 | scene | ✓✓ | ✓✓ | ✓ |
 | minimal | ✓✓ | ✓✓ | ✗ |
+
+## Font × Rendering
+
+| | flat-vector | hand-drawn | painterly | digital | pixel | chalk |
+|---|:---:|:---:|:---:|:---:|:---:|:---:|
+| clean | ✓✓ | ✗ | ✗ | ✓✓ | ✓ | ✗ |
+| handwritten | ✓ | ✓✓ | ✓✓ | ✓ | ✗ | ✓✓ |
+| serif | ✓ | ✗ | ✓ | ✓✓ | ✗ | ✗ |
+| display | ✓✓ | ✓ | ✓ | ✓✓ | ✓✓ | ✓ |