• Store kd-tree nodes in one contiguous std::vector instead of pointer-linked
  allocations, shrinking Node and speeding up kNN queries and builds
  (#319).
• Gather split coordinates once into a scratch array reused across builds,
  avoiding a second pass over the dataset in middleSplit_().
• Release node memory in freeIndex() and cap the node reservation at 2N.
• loadIndex() now validates the loaded node array and rejects corrupt files.
• Concurrent build: splice only the subtrees actually built by a task,
  avoiding unnecessary memmoves.
• Prefetch the right child while descending the tree, improving kNN
  query speed.
• CI Linux: do not upgrade runner packages before installing build tools
  (#320).
• Contributors: Jose Luis Blanco-Claraco, Luca Bartoli