Saturate Q15 variance results before narrowing

[Firat17aze]

Fixes #354.

arm_var_q15 documents that its 34.15 result is saturated to 1.15, but both the scalar and MVE paths assigned the shifted value directly to q15_t. A valid sample variance above 1.0 could therefore narrow to a negative result. This applies __SSAT(..., 16) before the assignment, matching the existing arm_std_q15 behavior.

Validation:

• Built the full library in Release mode with HOST=ON and DISABLEFLOAT16=ON using AppleClang 21.
• Ran a focused host reproducer at -O0 and at -O3 -ffast-math with loop unrolling. [0.75, -0.75] now saturates to 32767; an in-range case and the single-sample case remain unchanged.
• Linked and ran the same reproducer against the built libCMSISDSP.a.
• git diff --check passes.

No Arm hardware testing was performed, and the MVE path was not executed locally.

LLMs were used for review and code generation in some parts of this pull request.