Re-enable Downgrade CI

[ChrisRackauckas-Claude]

Summary

The Downgrade CI has been temporarily disabled in PR #291 to unblock CI while waiting for upstream dependency updates.

Action Items

• Monitor upstream dependencies (OrdinaryDiffEq, etc.) for compatibility updates
• Re-enable Downgrade CI once compatibility is resolved
• Remove the if: false condition from .github/workflows/Downgrade.yml

Related

• Similar to Re-enable downgrade tests after OrdinaryDiffEq updates DelayDiffEq.jl#356

[ChrisRackauckas-Claude]

Diagnosis from a downgrade-CI re-enablement pass (verified locally on Julia 1.10, isolated depot):

The downgrade suite fails because test/saving_tests.jl does using DiffEqBase: BrownFullBasicInit (used ~line 239), but BrownFullBasicInit was only added/exported in DiffEqBase v6.190.1 (v6.190.0 lacks it). The declared floor is DiffEqBase = "6.155.4, 7", so at the downgrade floor the symbol doesn't exist → UndefVarError: BrownFullBasicInit not defined in the Saving testset. The symbol is used only in the test, not in src/.

With these (genuine, minimal) resolution-floor bumps the rest of the suite passes (217 passed, 1 errored — the error being exactly the above):

• SciMLBase 2.54→2.69.0 (2.68.0/2.68.1 are themselves broken: LinearAliasSpecifier inner constructor calls new(...) with 4 args against a 2-field struct → precompile error)
• DiffEqBase 6.155.4→6.160.0, ADTypes 1.9.0→1.11.0, RecursiveArrayTools 3.27→3.27.2, StaticArraysCore 1.4→1.4.2, PrecompileTools 1.2→1.2.1

To clear the BrownFullBasicInit error, DiffEqBase must be ≥6.190.1, which forces SciMLBase ≥2.115 → RecursiveArrayTools ≥3.35 → LinearSolve 3.x → NonlinearSolve 4.x → OrdinaryDiffEq* 2.x — a coordinated stack-floor raise that narrows user-facing compat well beyond what src/ needs. Maintainer decision: either (a) raise the stack floors (accept the narrower compat), or (b) change the test to not depend on BrownFullBasicInit at the supported floor. No PR opened to avoid forcing (a) unilaterally.

From the SciML downgrade re-enablement pass.