[Biug] Fix tolerance checks for IEEE-754 zero handling

[dacozai]

Fix tolerance checks for IEEE-754 zero handling

Issue: #25

Rust f32/f64 follow IEEE-754, which defines both +0.0 and −0.0.
Because the previous code used is_positive() / is_negative(), values like +0.0 incorrectly passed checks that were intended to reject zero.

This PR updates all tolerance validations to use numeric comparisons instead of sign-bit checks:

• Parameters documented as “panic if < 0” now use:

  assert!(x >= F::zero())

• Parameters documented as “panic if ≤ 0” now use:

  assert!(x > F::zero())

This makes behaviour consistent with the documentation and eliminates IEEE-754 sign-bit edge cases.