fix: advance indices in unrolled loops of portable dcopy and scopy

[mantrapatel05]

Description

This PR fixes a unit-stride loop bug in the portable C implementations of blas/base/dcopy and blas/base/scopy. The unrolled loop copies seven elements per pass but did not advance ix and iy, so copies spanning multiple blocks repeated the first block and left later destination elements unchanged.

The loop now advances both indices by M after each pass. Regression tests cover lengths 14, 15, 16, 21, and 28 with non-zero offsets in both packages.

Related issue

Resolves #15823.

Checklist

• Read, understood, and followed the contributing guidelines

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When authoring the changes proposed in this PR, did you use any kind of AI assistance?

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Testing

• Compiled the portable C sources with GCC using -std=c99 -Wall -Wextra -Werror -O2; the direct harness passed for lengths 1 through 150 with non-zero offsets.
• Ran the filtered DCOPY and SCOPY package suites; their JavaScript tests passed.
• Native addon tests were skipped because the Windows C++ build tools required by node-gyp are not installed here.

On Linux and macOS, the native addon uses the Fortran implementation, so those builds do not exercise the portable C code changed here.

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Coverage Report

Package	Statements	Branches	Functions	Lines
blas/base/dcopy	$\\color{green}414/414$ $\\color{green}+100.00\\%$	$\\color{green}29/29$ $\\color{green}+100.00\\%$	$\\color{green}4/4$ $\\color{green}+100.00\\%$	$\\color{green}414/414$ $\\color{green}+100.00\\%$
blas/base/scopy	$\\color{green}414/414$ $\\color{green}+100.00\\%$	$\\color{green}29/29$ $\\color{green}+100.00\\%$	$\\color{green}4/4$ $\\color{green}+100.00\\%$	$\\color{green}414/414$ $\\color{green}+100.00\\%$

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