Restore sublibrary project metadata before downgrade QA

[ChrisRackauckas-Claude]

What changed and why

The julia-downgrade-compat action now promotes old-style [targets].test extras into [deps] so Pkg.test(; allow_reresolve=false) can retain their locked versions. For QA groups, Aqua consequently inspects the action-mutated project rather than the package's source metadata: it reports every promoted test extra as stale, and ModelingToolkitBase's promoted ModelingToolkitStandardLibrary/DiffEqNoiseProcess dependencies create a precompile cycle that fails the persistent-tasks check.

Save each sublibrary's original Project.toml before downgrade resolution and restore it after the downgraded build but before QA. This matches the existing root downgrade workflow: the build remains pinned to the downgraded manifest, while metadata-oriented QA checks inspect the source project rather than a transient action mutation.

Please ignore this draft until it has been reviewed by @ChrisRackauckas.

Regression evidence

The new workflow-order regression test failed on the unfixed workflow:

sublibrary downgrade restores project metadata for QA | 2 failed / 2 total
Expression: all(!isnothing, positions)
Expression: count("if: \"${{ inputs.group-env-value == 'QA' }}\"", txt) == 2
Evaluated: 0 == 2

After the workflow change, the same test passes:

sublibrary downgrade restores project metadata for QA | 3 passed / 3 total

I also reproduced the current ModelingToolkitBase failure locally from clean detached ModelingToolkit master e91f13e48fc4f2fdda67e66ad7b2d9c67234ac2b using Julia 1.10.12 and the exact downgrade action revision used by CI (30b83cf94e89744fcbd7ea5496be4967d6fbe540). Before restoring lib/ModelingToolkitBase/Project.toml, QA reported 17 passed, 2 failed (Stale dependencies and Persistent tasks). Restoring the source Project.toml before the same locked Pkg.test invocation produced:

JET Tests  | 54 passed / 54 total
Aqua Tests | 19 passed / 19 total
Testing ModelingToolkitBase tests passed

The same failure is present on current ModelingToolkit master in https://github.com/SciML/ModelingToolkit.jl/actions/runs/33444272942/job/99729331320. The behavior was introduced by julia-actions/julia-downgrade-compat@30b83cf.

Verification

julia +1.12 --startup-file=no test/runtests.jl
# all testsets passed; new regression test: 3/3

julia +1.12 --startup-file=no --project=@runic -m Runic --check --diff test/runtests.jl
# exit 0

typos .github/workflows/sublibrary-downgrade.yml test/runtests.jl
# exit 0

actionlint .github/workflows/sublibrary-downgrade.yml
# exit 0

git diff --check
# exit 0

Not verified before opening: a live downstream invocation of this reusable-workflow branch. The local ModelingToolkitBase run above exercises the same save/downgrade/build/restore/test behavior.

🤖 Generated with Codex CLI 0.151.0 (model: gpt-5.6-sol; session: 01a059b1-5fa2-76a2-937a-33502aa35de5).