CI and Release download Zig 0.16.0 from ziglang.org on every run; a slow mirror fails every Linux job

[0thernet]

Summary

Every Linux CI shard and the Release workflow's Linux build job fetch zig-x86_64-linux-0.16.0.tar.xz (55 MB) from https://ziglang.org/download/... with curl --max-time 180 --retry 2. On 2026-09-17 around 07:00 UTC the mirror served roughly 7 MB per minute, so all seven Ubuntu shards on PR #244 failed twice with curl: (28) Operation timed out after 180000 milliseconds while every macOS shard passed. The same step in .github/workflows/release.yml (line 135) would fail a release run the same way.

Expected

The Zig toolchain should come from a source the repository controls or caches: actions/cache keyed on the pinned SHA-256, a GitHub Release asset in a repository-owned mirror, or a vendored artifact. The SHA-256 check should stay exactly as it is. A slow upstream mirror should not fail governed CI or a release.

Evidence

• PR docs(devin): record the first live acceptance turn (Phase 4) #244 run 35195913507, attempts 1 and 2: Ubuntu shards fail in "Download pinned Zig 0.16.0 for authority supervisor (Linux)"; macOS passes.
• A ranged 20 MB fetch from a residential connection at the same time took about 60 s for 7.3 MB.