no_std migration

[day01]

Motivation

The initial no_std work establishes a working bare-metal baseline for the core crate, but it does not make the entire API available without std.

Several modules are still gated behind std even though they primarily require allocation rather than operating-system functionality. The nalgebra feature also currently enables std unconditionally.

Scope

• support core-only bare-metal builds
• support floating-point operations through libm
• migrate errors to core::error::Error
• support rand without std
• add bare-metal CI checks
• introduce an alloc feature
• move allocation-based APIs from std to alloc
• migrate Categorical, Empirical and allocation-based statistics
• make nalgebra work with no_std + alloc
• verify representative calculations in a real no_std environment
• document the supported feature combinations

Known limitation

KDE remains std-only because its kdtree dependency requires std.

PRs

• Add initial no_std support #399
• Support alloc-backed APIs in no_std #403