Add built-in host alias for box-to-host access

[DorianZheng]

Summary

• add a built-in host.boxlite.internal alias that resolves inside the guest to a dedicated host IP
• route that host IP to host loopback through gvproxy and keep the alias reachable under restrictive allow_net
• add Go and Rust coverage for DNS ordering, NAT behavior, TCP filtering, and VM-level host alias behavior
• replace the Docker-specific host.docker.internal guidance with BoxLite-specific docs and examples

Why

The docs previously referenced host.docker.internal, but BoxLite did not actually implement or document a BoxLite-native host alias in the runtime. This change makes box-to-host loopback access explicit, test-covered, and documented in the codebase.

User impact

Boxes can now reach host loopback-backed services through host.boxlite.internal or the dedicated virtual host IP 192.168.127.254 when networking is enabled. The docs also now distinguish host-to-box port forwarding from box-to-host connectivity.

Validation

• cargo fmt --check
• cargo test -p boxlite --features gvproxy --lib net::gvproxy::config
• cargo clippy -p boxlite --features gvproxy --tests -- -D warnings
• go test ./... in src/deps/libgvproxy-sys/gvproxy-bridge

Notes

• the tree includes ignored VM integration tests covering host alias resolution and host loopback reachability
• the branch was published from the current local commit contents with a clean main base