Add ARM SVE backend stub

[unamedkr]

The CPU backend (src/backend/cpu/) has optimized paths for NEON and AVX2, but no SVE (Scalable Vector Extension) support. SVE is needed for AWS Graviton3/4 and other modern ARM servers.

What to do:

• Create src/backend/cpu/tq_sve.c with stub implementations of the key kernel functions (quantize, dequantize, dot product).
• Each stub should simply call the existing generic C fallback — this sets up the scaffolding for future SVE intrinsics.
• Add SVE feature detection in the backend dispatch logic and wire up the stubs.
• Guard with #if defined(__ARM_FEATURE_SVE).

Files to touch: src/backend/cpu/tq_sve.c (new), src/backend/cpu/tq_dispatch.c or equivalent dispatch file.