[decimal] Add rational root decompostion to root() and add atanh approach to ln()

[forfudan]

This PR adds two performance optimizations to BigDecimal's exponential functions: rational root decomposition for fractional roots and a hybrid Taylor/atanh approach for the natural logarithm series.

Changes:

• Added rational root decomposition for root() to compute fractional roots like x^(2/3) using integer_power(integer_root(x, a), b) instead of the expensive exp(ln(x)/n) path
• Refactored ln_series_expansion() to use atanh identity for large z values, reducing iterations by ~3× through faster convergence (rate 1/9 vs 1/2)
• Fixed markdown table-of-contents comment syntax from "omit in toc" to "omit from toc"

[Copilot]

Pull request overview

This PR adds two performance optimizations to BigDecimal's exponential functions: rational root decomposition for fractional roots and a hybrid Taylor/atanh approach for the natural logarithm series.

Changes:

• Added rational root decomposition for root() to compute fractional roots like x^(2/3) using integer_power(integer_root(x, a), b) instead of the expensive exp(ln(x)/n) path
• Refactored ln_series_expansion() to use atanh identity for large z values, reducing iterations by ~3× through faster convergence (rate 1/9 vs 1/2)
• Fixed markdown table-of-contents comment syntax from "omit in toc" to "omit from toc"

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/decimojo/bigdecimal/exponential.mojo	Added _gcd() helper, _rational_root_decomposition() function, integrated rational root decomposition into root(), and refactored ln_series_expansion() with hybrid Taylor/atanh approach
docs/readme_zht.md	Fixed markdown TOC comment syntax (spelling fix)
docs/readme_unreleased.md	Fixed markdown TOC comment syntax (spelling fix)
docs/readme_tomlmojo.md	Fixed markdown TOC comment syntax (spelling fix)
docs/plans/bigdecimal_biguint_benchmark_analysis.md	Updated task completion status for Task 3f (atanh for ln) and Task 7c (rational root decomposition)
README.md	Fixed markdown TOC comment syntax (spelling fix)

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