Re-enable downgrade (strict, allow_reresolve=false); intentionally RED pending upstream ForwardDiff/LogExpFunctions — red until ForwardDiff 1.3.4 (#811). saveat fix is separate (#600) — do NOT touch it — natural failure, not disabled; auto-greens on upstream release.

[ChrisRackauckas-Claude]

Summary

The IRKGL2026a rewrite (62e2fe2) deleted the in-place PolInterp! from src/IRKCoefficients.jl and rewrote the internal tcoeffs/tcache struct layout plus the GaussLegendreCoefficients!/EstimateCoeffs! signatures, but left test/alloc_tests.jl calling the old API. As a result the test suite was red at any compat: UndefVarError: PolInterp! not defined at the import line, and (once that is fixed) constructor/arity mismatches.

Changes

• Restore PolInterp! (in-place Lagrange interpolation writing into a preallocated buffer) in src/IRKCoefficients.jl, ported to the current code, so the zero-allocation test's intent holds.
• Port alloc_tests.jl to the rewritten internal API:
  • add the eta array (zeros(s, s)) to the tcoeffs constructor (now 13 fields),
  • call GaussLegendreCoefficients!(s, mu, c, b, eta, T) and EstimateCoeffs!(s, alpha, T) (new arities),
  • add the trailing verbose::DEVerbosity field to the tcache constructor.
• Fix the MyNorm allocation measurement via a function barrier. @allocated on a call closing over a non-const @testset variable measures the boxing of that captured variable, not the callee — MyNorm itself allocates 0 bytes (verified by measuring inside a function). This keeps the zero-allocation assertion meaningful; it is not weakened.
• Re-enable Downgrade.yml to RUN STRICT. Removed the if: false gating so the downgrade job is no longer skipped. It runs strict (allow_reresolve=false) — floors are NOT lowered and allow-reresolve is NOT added. The strict resolve currently cannot be satisfied (the floored DiffEqBase/NonlinearSolveBase stack pins LogExpFunctions 1.0.1, which caps ForwardDiff while the test extras require ForwardDiff 1.x — disjoint windows), so the job is intentionally RED. This is a natural, visible failure, not a disabled job. It will auto-green when the upstream fix releases (ForwardDiff/LogExpFunctions — red until ForwardDiff 1.3.4, #811).
• Bump Reexport floor 1.0 -> 1.2.2. Transitive BracketingNonlinearSolve requires Reexport >= 1.2.2; floors 1.0/1.1/1.2.0/1.2.1 fail to resolve.

Testing (Julia 1.10, local)

Full suite at latest compat: PASS

Test Summary:    | Pass  Total  Time
Allocation Tests |    6      6
     Testing IRKGaussLegendre tests passed

Runic formatting checked clean on the changed .jl files.

Ignore until reviewed by @ChrisRackauckas.

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Note re: the SciML/.github downgrade-workflow change (always allow_reresolve: false, removing the allow-reresolve workflow_call input): this PR's Downgrade.yml does not pass an allow-reresolve: input (it only references the term in the explanatory comment), so no edit is needed here — it stays compatible after @v1 is retagged.