feat: add 5 interactive playground scenarios

[godofecht]

Summary

• CartPole (RL): Physics simulation with cart+pole animation, bang-bang control policy, episode tracking
• Pong (RL): Ball physics with paddle AI, glow effects, trail rendering, score display
• CNN Convolution: 7x7 input grid with 4 switchable kernels (edge detect, sharpen, blur), animated step-by-step convolution, heatmap output
• Heat Equation (PINN): 30x30 grid with Laplacian diffusion, blue-to-red heatmap with color bar legend
• Traffic Flow (GNN): 10-node graph with message-passing diffusion, animated flow particles, congestion coloring

All scenarios run as JS simulations (no server required) with C++ server endpoints for future backend integration. Existing 4 models fully preserved.

Test plan

• CI passes (playground changes don't affect C++ build/tests)
• Open playground/index.html in browser, verify all 9 models appear in sidebar
• Click each scenario, hit "Run Inference", verify canvas animations render

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