Build-script tests

Integration tests of the build.rs glue (Build-script glue (FEAT_0083)). Live under crates/ethercat-netcfg-build/tests/.

Test Case: Build helper generates into OUT_DIR and the module compiles TEST_0839
status: open
verifies: REQ_0830

A throwaway consumer crate whose build.rs invokes the helper over a fixture compiles successfully while pulling the module in via include!(concat!(env!("OUT_DIR"), "/network.rs")). The generated file is confirmed absent from the source tree.
Test Case: Build helper emits rerun-if-changed for config and ESI TEST_0840
status: open
verifies: REQ_0831

Capturing the build-script stdout, the test asserts a cargo:rerun-if-changed line for the network.yaml and for each vendored ESI file the fixture references.