DowngradeSublibraries CI broken by julia-downgrade-compat@v2 removing local path deps from manifest

[ChrisRackauckas-Claude]

Problem

Since julia-actions/julia-downgrade-compat@v2 commit 066064d (Jan 21), all DowngradeSublibraries CI runs fail with:

ERROR: LoadError: could not find manifest entry for package with uuid bbf590c4-e513-4bbe-9b18-05decba2e5d8

That UUID is OrdinaryDiffEqCore.

Root cause

The updated julia-downgrade-compat action detects packages with local [sources] paths and temporarily removes them from [deps], [compat], and [sources] before running Resolver.jl. After resolution it restores the Project.toml, but the Manifest.toml is left without entries for these removed packages (OrdinaryDiffEqCore, OrdinaryDiffEqNonlinearSolve, OrdinaryDiffEqDifferentiation).

When julia-buildpkg then calls Pkg.build(), it fails because OrdinaryDiffEqCore is in the Project.toml deps but missing from the manifest.

Last successful run: 21202786425 (Jan 21 08:39, before the upstream change was picked up).

Possible fixes

1. Upstream fix in julia-downgrade-compat: After restoring the Project.toml, run Pkg.resolve() to re-add the local path deps to the manifest.
2. Workaround: Remove julia-buildpkg step and set allow_reresolve: true on julia-runtest so it re-resolves the manifest before running tests.
3. Workaround: Add an explicit Pkg.resolve() step between julia-downgrade-compat and julia-buildpkg.

Current status

DowngradeSublibraries has been disabled via if: false in #3020 pending resolution.

Upstream issue should be filed at https://github.com/julia-actions/julia-downgrade-compat.