refactor(baoyu-cover-image): replace 20 styles with 5D palette×rendering system

Migrate from 4-dimension (Type×Style×Text×Mood) to 5-dimension
(Type×Palette×Rendering×Text×Mood) system. 9 palettes × 6 renderings
= 54 combinations replacing 20 fixed styles. Add style presets for
backward compatibility, v2→v3 schema migration, and new reference
structure (palettes/, renderings/, workflow/).

skills/baoyu-cover-image/references/renderings/flat-vector.md

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+# flat-vector
+
+Clean, modern, geometric illustration
+
+## Core Characteristics
+
+Flat design with clean outlines, uniform fills, and no texture or depth. Think modern app icons, infographic illustrations, and vector-based editorial art.
+
+## Lines
+
+- Clean outlines with uniform stroke weight
+- Closed shapes (coloring-book style)
+- Rounded line endings, avoid sharp corners
+- Consistent stroke width throughout
+
+## Texture
+
+- None: smooth, flat color fills only
+- No gradients, shadows, or noise
+- Solid color blocks
+
+## Depth
+
+- Flat: no shadows, no perspective
+- 2D layering with overlap for depth illusion
+- Optional 2.5D isometric layering (front/back occlusion, no atmospheric perspective)
+- No 3D effects or bevels
+
+## Element Vocabulary
+
+- Geometric icons and simple shapes
+- Bold outlined objects with clean fills
+- Geometric simplification: complex objects → basic shapes (trees → lollipop/triangle, buildings → rectangles)
+- "Toy model" aesthetic: cute, rounded proportions
+- Decorative: dots, lines, sunbursts, pill-shaped clouds, small stars
+- Isolated elements on clean backgrounds
+
+## Typography Approach
+
+- Clean sans-serif or bold geometric lettering
+- Strong readability, consistent weight
+- Easily scalable at any size