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/chalk.md

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+# chalk
+
+Educational, authentic, classroom
+
+## Core Characteristics
+
+Chalk on blackboard aesthetic with imperfect strokes, dust effects, and authentic classroom feel. Nostalgic educational warmth.
+
+## Lines
+
+- Imperfect chalk strokes with variable pressure
+- Visible chalk texture and grain
+- Slightly wobbly, hand-drawn quality
+- Thick strokes for emphasis, thin for details
+
+## Texture
+
+- Chalk dust effects around text and elements
+- Board surface (dark, slightly worn)
+- Eraser smudges and residue
+- Grainy chalk quality on all elements
+
+## Depth
+
+- None: flat chalk drawings on board surface
+- Layering through erasure and redrawing
+- No shadows or perspective
+
+## Element Vocabulary
+
+- Chalk doodles: stars, arrows, underlines
+- Mathematical formulas and diagrams
+- Stick figures and simple icons
+- Connection lines with chalk feel
+- Checkmarks, circles, boxes for lists
+- Wooden frame border optional
+
+## Typography Approach
+
+- Hand-drawn chalk lettering
+- Imperfect baseline, authentic classroom feel
+- White or bright colored chalk for emphasis
+- Variable sizing for hierarchy

skills/baoyu-cover-image/references/renderings/digital.md

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+# digital
+
+Polished, precise, modern
+
+## Core Characteristics
+
+Clean digital illustration with polished finish, precise edges, and subtle modern effects. Feels like a professional UI mockup or corporate illustration.
+
+## Lines
+
+- Clean, precise, computer-perfect edges
+- Consistent stroke weights
+- Sharp corners where appropriate
+- Anti-aliased smooth rendering
+
+## Texture
+
+- Smooth surfaces with no visible texture
+- Subtle gradients permitted (soft, controlled)
+- Frosted glass and blur effects
+- Clean shadows with consistent direction
+
+## Depth
+
+- Subtle gradients and soft drop shadows
+- Layered card-based layouts
+- Light 3D effects (subtle, not realistic)
+- Material Design-inspired elevation
+
+## Element Vocabulary
+
+- Polished icons and UI components
+- Data visualizations: charts, graphs, metrics
+- Card layouts and structured grids
+- Tag chips, progress bars, status indicators
+- Clean geometric shapes
+
+## Typography Approach
+
+- System UI or modern sans-serif (Inter, SF Pro style)
+- Clean, functional, high readability
+- Structured hierarchy with consistent spacing

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

skills/baoyu-cover-image/references/renderings/hand-drawn.md

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+# hand-drawn
+
+Sketchy, organic, personal
+
+## Core Characteristics
+
+Hand-drawn illustration with visible imperfections, organic line quality, and personal touch. Feels like a skilled artist's sketchbook or whiteboard drawing.
+
+## Lines
+
+- Sketchy, organic, slightly imperfect strokes
+- Variable line weight (thicker at pressure points)
+- Wavy connectors and arrows
+- Natural hand tremor visible
+
+## Texture
+
+- Paper grain and subtle surface texture
+- Pencil/pen/marker texture on strokes
+- Casual fills with visible brush direction
+
+## Depth
+
+- Minimal: light hand-drawn shadows or hatching
+- No realistic depth or perspective
+- Simple layering with overlap
+
+## Element Vocabulary
+
+- Doodles, organic shapes, hand-lettered labels
+- Conceptual icons with sketchy quality
+- Connection lines with hand-drawn wavy feel
+- Stars, arrows, underlines, circles, checkmarks
+- Stick figures and simple characters
+
+## Typography Approach
+
+- Hand-lettered or marker-style text
+- Bouncy baselines, organic feel
+- Variable sizes for emphasis hierarchy

skills/baoyu-cover-image/references/renderings/painterly.md

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+# painterly
+
+Soft, artistic, expressive
+
+## Core Characteristics
+
+Watercolor or paint-style illustration with visible brush strokes, color bleeds, and artistic texture. Feels like a hand-painted art piece.
+
+## Lines
+
+- Soft brush strokes with variable opacity
+- No hard outlines; edges defined by color transitions
+- Organic flowing strokes with natural blending
+
+## Texture
+
+- Visible paint or watercolor wash textures
+- Color bleeds and wet-on-wet effects
+- Paper texture showing through transparent areas
+- Brush stroke patterns visible
+
+## Depth
+
+- Soft edges with natural color blending
+- Atmospheric depth through color fading
+- Layered washes creating depth illusion
+
+## Element Vocabulary
+
+- Watercolor washes as backgrounds
+- Natural elements: leaves, flowers, organic forms
+- Soft gradients and color transitions
+- Splatter and drip effects as accents
+- Botanical and environmental motifs
+
+## Typography Approach
+
+- Elegant brush script or handwritten style
+- Organic letterforms with brush texture
+- Integrated with paint environment

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