Repair the stale process helper import in the Linux amd64 case suite

[LunaStev]

Observed on PR #606 head 037baf248a9793be2b0563fb28972b5c4fb9714a (2026-09-12).

The process remapping change in #606 removes _proc_dup_child_fd, but tests/cases/linux/amd64/test13.wave still imports it. This is a regression introduced by the PR, separate from the descriptor remapping algorithm itself.

Failing Linux amd64 job, step Check every target case:

error[E3001]: module 'std::process::spawn' has no symbol '_proc_dup_child_fd'
.../tests/cases/linux/amd64/test13.wave:1:1

The failure prevents the Linux runtime case runner from starting. Repository search finds the obsolete name in this case's import list, with no call to it.

A read-only local audit checked all 605 manifest sources using the current checkout's std library: 604 passed and this case was the only failure. The audit continued after errors, unlike the current CI precheck.

Starting points:

• test13.wave import list
• process spawning implementation

Reproduce with the PR's compiler and std library installed from the same checkout:

wavec check tests/cases/linux/amd64/test13.wave

Acceptance:

• Reconcile the import with the revised helper surface (remove the unused import, or preserve a wrapper if compatibility requires it).
• Preserve the existing process test body and the new descriptor-remapping regression; do not delete or skip the case.
• Check every manifest-selected source, then run the Linux amd64 cases to expose any later failures.

Related: #558. This is a small import integration fix suitable for a first contribution.