Support KDE in no_std

[day01]

Motivation

KDE is currently gated by std because kdtree does not expose its allocation-backed API in no_std builds. This continues the no_std work tracked in #400 and integrates the upstream support from mrhooray/kdtree-rs#92.

Changes

• enable kde with no_std + alloc through the upstream kdtree libm feature
• import num_traits::Float where density calculations need floating-point methods without std
• compute the neighborhood radius from the farthest returned distance instead of assuming kdtree::within result ordering
• add a bare-metal KDE check and execute kde_pdf and knn_pdf in the existing no_std Wasm runtime test
• document the supported no_std feature combination

Blocked by

• merge and release mrhooray/kdtree-rs#92
• replace the temporary Git revision with the released crates.io version before merge

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.07%. Comparing base (92819b6) to head (e9d0baa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   95.07%   95.07%           
=======================================
  Files          62       62           
  Lines       14191    14191           
=======================================
  Hits        13492    13492           
  Misses        699      699           

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