Update kdtree and fix KDE radius

[day01]

Motivation

kdtree 0.8.1 intentionally changed within() to return unordered results in mrhooray/kdtree-rs#85. The existing KDE and KNN implementations used the last result as the neighborhood radius, which produces incorrect densities after the dependency update.

Changes

• update kdtree from 0.7.0 to 0.8.1
• compute the neighborhood radius from the farthest returned distance instead of relying on result order
• add a regression test with deliberately unordered distances
• update the MSRV lockfile

Refs:
#428

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.07%. Comparing base (92819b6) to head (edff6a8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #444   +/-   ##
=======================================
  Coverage   95.07%   95.07%           
=======================================
  Files          62       62           
  Lines       14191    14191           
=======================================
  Hits        13492    13492           
  Misses        699      699           

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