Propagate failed NNUE downloads from net.sh
When both NNUE download mirrors fail (or both return invalid content), `fetch_network` returns 1, but the following embedding `if` statement overwrites that status. Consequently, `make net` reports success; the universal-build path with argument `0` also creates an empty network dump.
Exit immediately when fetching the network fails. The PR contains this one-line fix and the first-time contributor entry required by CONTRIBUTING.md. The offline regression test and its workflow step were removed as requested in review; the test was retained outside the repository for local verification.
Validation:
- Local offline regression checks cover failed downloads and invalid checksums in all three invocation modes, fallback downloads, cached files, and successful primary downloads. They failed against the original script and pass against the final fix.
- `sh -n scripts/net.sh` and `git diff --check`
- Real `make net` download and checksum validation; cached-network reuse rechecked after the revision
- Clean Apple silicon build with Clang and Xcode's macOS 26.4 SDK before the test removal; engine sources are unchanged by the revision
- `tests/signature.sh 1559611` rechecked after the revision
Prepared with OpenAI Codex. A second Codex agent independently reviewed the original change and reproduced the regression against the base revision, with no findings.
closes https://github.com/official-stockfish/Stockfish/pull/7232
No functional change