feat(lib): add stats/incr/nanmmax

[ivishal-g]

Resolves #5584.

Description

This pull request adds a new incremental statistics package @stdlib/stats/incr/nanmmax, which provides an accumulator function that computes a cumulative maximum while correctly ignoring NaN values (similar to nanmin, nansum, etc.).

The implementation properly handles signed-zero edge cases and returns null until the first non-NaN value is observed.

Related Issues

This pull request resolves #5584.

Questions

No.

Other

No.

Checklist

• [x] Read, understood, and followed the [contributing guidelines][contributing].

---

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/incr/nanmmax	$\color{green}132/132$ $\color{green}+100.00%$	$\color{green}14/14$ $\color{green}+100.00%$	$\color{green}2/2$ $\color{green}+100.00%$	$\color{green}132/132$ $\color{green}+100.00%$

The above coverage report was generated for the changes in this PR.