refactor: use shared Iterator type in iter/cugmean

[Planeshifter]

Description

What is the purpose of this pull request?

This pull request replaces the local Iterator union type (Iter or IterableIterator) with the direct import { Iterator } from "@stdlib/types/iter" used by all eight sibling iter/cu*mean packages (e.g. cumean), which share an identical declaration shape and type test.

This is part of a series of follow-up PRs addressing findings from a TypeScript declaration audit of the stats namespace (documentation-only fixes landed separately in #12482).

Related Issues

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No.

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Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

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Disclosure

This change was surfaced by a Claude Code audit of the stats namespace TypeScript declarations. The fix was verified against the implementation and declaration test before submission, and reviewed by myself.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/iter/cugmean	$\color{green}198/198$ $\color{green}+100.00\%$	$\color{green}18/18$ $\color{green}+100.00\%$	$\color{green}4/4$ $\color{green}+100.00\%$	$\color{green}198/198$ $\color{green}+100.00\%$

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

[kgryte]

This isn't quite what we want. We can actually improve type specificity here and in other similar packages in a few ways:

1. The union is correct insofar as the function accepts both iterators and iterable iterators.
2. The output iterator should be typed, as it always returns numbers.
3. We should likely type the input iterator, as well, since it should return numbers.
4. Via the union, we can use generics where T extends the union and we return T. That should cover both the iterator and iterable iterator behavior mentioned in the notes for those environments supporting Symbol.iterator.