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## Problem Previously, when cutting over to cold secondary locations, we would clobber the previous, good, heatmap with a cold one. This is because heatmap generation used to include only resident layers. Once this merges, we can add an endpoint which triggers full heatmap hydration on attached locations to heal cold migrations. ## Summary of changes With this patch, heatmap generation becomes additive. If we have a heatmap from when this location was secondary, the new uploaded heatmap will be the result of a reconciliation between the old one and the on disk resident layers. More concretely, when we have the previous heatmap: 1. Filter the previous heatmap and keep layers that are (a) present in the current layer map, (b) visible, (c) not resident. Call this set of layers `visible_non_resident`. 2. From the layer map, select all layers that are resident and visible. Call this set of layers `resident`. 3. The new heatmap is the result of merging the two disjoint sets. Related https://github.com/neondatabase/neon/issues/10541