Gradient Boosting 2D: Residual Heatmap Across Rounds

This is the 2D companion to the 3D gradient-boosting simulator, computed independently rather than rendered as a flattened camera angle on the same scene. It runs the identical algorithm — a real greedy, SSE-minimizing regression tree fit to residuals every round, added back with shrinkage — but shows it through two grid-native panels: a scrolling round-by-point residual heatmap that cools from red/blue toward white as the ensemble learns, and a live strip of the most recent tree's x-partition with its per-leaf residual bars. Shrinkage, tree depth and dataset noise remain live controls, so the same bias/variance trade-off is visible directly in how fast — and how evenly — the heatmap converges.