fix: update fft/base/fftpack/rffti for correct twiddle factor placement

[gunjjoshi]

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Resolves stdlib-js/metr-issue-tracker#961.

Description

What is the purpose of this pull request?

This pull request:

• updates fft/base/fftpack/rffti to write twiddle factors starting from index N, instead of N+1.
• this is due to match the FFTPACK behavior, as observed in [RFC]: refactor fft/base/fftpack/rffti to fix twiddle factor indices metr-issue-tracker#961 (comment).

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: refactor fft/base/fftpack/rffti to fix twiddle factor indices metr-issue-tracker#961.

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No.

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No.

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Used gpt-5.4-mini to review changes and to test FFTPACK's C implementation.

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Coverage Report

Package	Statements	Branches	Functions	Lines
fft/base/fftpack/rffti	$\\color{green}412/412$ $\\color{green}+0.00\\%$	$\\color{green}12/12$ $\\color{green}+0.00\\%$	$\\color{green}2/2$ $\\color{green}+0.00\\%$	$\\color{green}412/412$ $\\color{green}+0.00\\%$

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

[gunjjoshi]

The failing tests are because fft/base/fftpack/cosqi uses fft/base/fftpack/rffti. Once this is merged, I'll update cosqi accordingly. Or, should I add the changes in this PR itself?

[kgryte]

Separate PR.

[kgryte]

Is this check even necessary anymore? Wasn't this to confirm the buggy behavior?

[gunjjoshi]

This check is necessary for a slight different reason. The rffti function shouldn't touch the scratch space (index 0 to index N-1)(Ref). So, through this, we are making sure that the scratch space is kept untouched, and the twiddle factors are not written inside the scratch space.

[kgryte]

This looks good to me, except for some minor test clean-up.