Enable Snapdragon backend support in Rust llama.cpp integration

llama.cpp now has an experimental Snapdragon backend for:

• Android
• Windows on Snapdragon

Docs:

• README
• Windows setup

Rust-side support for this backend can be added, including:

• Cargo/build feature wiring for Snapdragon backends (GGML_OPENCL, GGML_HEXAGON)
• runtime device/backend selection where needed
• target support for Android arm64 and Windows arm64

This is Snapdragon-specific, and the Windows Hexagon path has additional SDK/driver/signing requirements per upstream docs.