v6.0.5

Build Rust NIF via Makefile for precompilation

- Add Rust NIF (tms_engine) build to Makefile with cross-compilation
  support via RUST_TARGET env var
- Set skip_compilation?: true in Rustler to prevent cargo fallback
- Include native/tms_engine.* in precompiled tarball priv_paths
- Pass RUST_TARGET from CI matrix to precompile step
- Bump to v6.0.5

Both NIFs (C++ prometheus_nif + Rust tms_engine) are now built by
elixir_make and packaged in the same precompiled artifact. End users
no longer need Rust or a C++ compiler installed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>