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/pixel.md

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+# pixel
+
+Retro 8-bit, nostalgic, chunky
+
+## Core Characteristics
+
+Pixel art aesthetic with visible pixel grid, limited color palette, and nostalgic gaming feel. Emulates classic 8-bit and 16-bit era graphics.
+
+## Lines
+
+- Pixel grid alignment, no anti-aliasing
+- Staircase edges on diagonals
+- Single-pixel or double-pixel outlines
+- Blocky, angular forms
+
+## Texture
+
+- Dithering patterns for gradients
+- No smooth transitions
+- Cross-hatching with pixel precision
+- Limited 16-32 color palette per scene
+
+## Depth
+
+- None: flat pixel planes only
+- Parallax layering (foreground/background)
+- No perspective or 3D effects
+
+## Element Vocabulary
+
+- 8-bit sprites and chunky shapes
+- Simple iconography: stars, hearts, arrows
+- Text bubbles with pixel borders
+- Progress bars with chunky segments
+- Retro gaming UI elements
+
+## Typography Approach
+
+- Pixelated bitmap font style
+- Chunky blocky letterforms
+- Fixed-width or monospace feel
+- All-caps for headers