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

@@ -0,0 +1,43 @@
+# 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