test(core,server): make the suite pass as root, under a slow init, and on Linux rustdoc

[RutaTang]

Summary

Running the backend CI gate (clippy, rustdoc with warnings denied, the hermetic suite for clew-core, clew-server and clew-protocol) on a Linux container turned up four failures the macOS-hosted jobs never meet. All four fixes are in tests and docs; no product logic changes.

• Two tests fail as root. statefile::tests::a_store_whose_read_failed_is_not_a_refusal and clew-server's state::tests::a_refused_merge_and_a_failed_one_are_told_apart observe a failed read or write through chmod, which root reads and writes past. They now return early as root, through a new shared testutil::running_as_root(), matching what the fs_scan tests already do inline.
• The cancel-while-forking test fails under a slow-reaping init. lsp::store::tests::a_cancel_while_the_installer_forks_stops_every_helper probed the process group once, 100 ms after the kill. Killed helpers linger as zombies until init reaps them (a container without an init does so slowly), and Linux counts a zombie as signalled. The probe now waits up to ten seconds, as the procgroup tests already do; a real survivor is a sleep 30, well past that.
• settle_new_executable hits Linux's ETXTBSY in a parallel suite. A child that another test thread forked while the script was still open for writing holds that descriptor until it execs, and exec of the file fails meanwhile. The helper now retries that one error briefly; absorbing first-run costs is what it exists for.
• A doc link breaks rustdoc on Linux. net.rs linked ProxyConfig::from_system, which is only built on macOS or under test, so cargo doc -D warnings failed on Linux. It is plain code text now.

Verification

On Linux, for clew-core, clew-server and clew-protocol:

Check	Result
scripts/hermetic-test.sh	808 passed, 0 failed
cargo clippy --all-targets -- -D warnings	0 warnings
cargo doc --no-deps --document-private-items with -D warnings	passes
cargo fmt --all -- --check	passes

🤖 Generated with Claude Code

https://claude.ai/code/session_01WRJ4sDknvQJ86tFvGydgpf

---

Generated by Claude Code