Add SVG visualizations to 11 challenge descriptions

[kunal-mansukhani]

Summary

• Adds inline SVG diagrams to 11 challenge HTML files where visualization significantly aids understanding
• Uses a consistent dark theme (#222 background, #ccc text, blue/green accent colors) matching the existing GPT-2 block visualization
• All SVGs are static, compact, and placed after the problem description paragraph

Challenges updated

Challenge	Visualization
1D Convolution	Kernel sliding over input array with dot product computation
2D Convolution	Kernel overlay on 4×4 grid with element-wise multiply-add breakdown
2D Max Pooling	Color-coded 2×2 pool regions with max values extracted to output
Matrix Transpose	Color-coded row→column movement with curved dashed arrows
Prefix Sum	Input→output with accumulation arrows showing growing dependency
Reduction	Binary tree reduction: 8 values → 4 → 2 → 1 final sum
Causal Attention	6×6 triangular attend/masked grid with checkmarks and -∞ symbols
Sliding Window Attention	8×8 banded attention pattern with window_size=2
BFS Shortest Path	5×5 grid with obstacles, start/end cells, and numbered shortest path
Jacobi Stencil 2D	5-point stencil with labeled neighbors and arrows to center cell
Interleave	Two input arrays with crossing arrows to alternating output positions

Test plan

• Verify SVGs render correctly in the browser on the challenge pages
• Check that no existing content was altered (only insertions)
• Confirm dark theme consistency across all 11 visualizations

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