Implement log2

[TimTheBig]

This patch implements log2 in the style of log10

[gemini-code-assist]

Code Review

This pull request implements the base-2 logarithm (log2) for the Real type, introducing a new Log2 symbolic class and the Real::log2 method. The implementation includes optimizations for rational powers of two and a division shortcut that folds ln(x) / ln(2) into a Log2 certificate. Support for log2 and lg was also added to the expression parser, along with comprehensive tests and benchmarks. A critical issue was identified in the division folding logic where cloning the original Real object incorrectly preserves a stale approximation cache, which could lead to incorrect numerical results.

[TimTheBig]

thanks