feat(hip): add vector filter backend

[tomatotomata]

Summary

• add the CeedVectorFilter host and HIP-device implementations to /gpu/hip/ref
• register the backend operation so the existing public API uses the HIP path
• keep the threshold semantics aligned with the existing CUDA reference backend

This follows the remaining HIP work identified in #1830 and ports the already-merged CUDA implementation from #1988. The existing tests/t129-vector.c covers the public operation for vector sizes including zero and threshold boundary cases.

Validation

• git diff --check passed
• local HIP compilation was not available because this Windows checkout has neither hipcc nor GNU make; the HIP source will need the repository CI environment for compile and test validation

LLM/GenAI disclosure

I used Codex to help inspect the existing CUDA implementation and draft the HIP adaptation. I reviewed the resulting diff and am responsible for the submitted code.

[jeremylt]

The associated issue is intended for students to work on - not mass automated submissions