[efegokdemir]

Implements the sgn item in #144.

Summary

• Add sgn to core::numbers.
• Return -1, 0 or 1 for negative, zero and positive quantities.
• Accept dimensioned quantities and return a dimensionless Scalar.
• Preserve NaN and handle positive and negative infinity.
• Add regression tests for scalars, physical units and special values.

Validation

• Numbat mathematical function tests.
• Full numbat package test suite.
• Formatting and diff checks.
• Clippy with only the existing upstream map_or_identity warning excluded.

This PR implements only the sign-function item. Issue #144 should remain open for its other tasks.