refactor(ci): use native runners for cross-platform builds

- Replace Docker-based cross-compilation with native GitHub runners
- Linux x86_64: ubuntu-latest (native)
- Linux ARM64: ubuntu-latest + cross-rs (proven approach from release.yml)
- Windows: windows-latest with MSVC (native)
- macOS: macos-13 (Intel) and macos-latest (ARM64) - skipped by default
- Remove broken Docker containers that couldn't cross-compile llama.cpp
- This matches the working approach in release.yml

Author: Michael-A-Kuykendall