Support alloc-backed APIs in no_std

[day01]

Motivation

This PR continues the work from #400 by extending no_std support to APIs that require heap allocation.

Types such as Vec and BTreeMap are provided by Rust's alloc crate and do not inherently require std. Using alloc allows these APIs to work in no_std.

Changes

• migrate statrs-owned heap allocations from std to alloc
• make Categorical, Empirical, log_spaced, ranks, and statistical tests available in no_std
• enable matrix-backed distributions with nalgebra using its alloc and libm support
• document allocator setup for no_std applications
• add bare-metal checks for core-only, rand, nalgebra, and nalgebra,rand
• add executable Wasm verification covering representative calculations

KDE cannot be migrated because its kdtree dependency requires std.

Verification

executable no_std calculations on wasm32-unknown-unknown

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.77%. Comparing base (1029458) to head (d7a0177).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #403   +/-   ##
=======================================
  Coverage   94.77%   94.77%           
=======================================
  Files          61       61           
  Lines       13402    13402           
=======================================
  Hits        12702    12702           
  Misses        700      700           

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