Add downstream ODE regression test for immutable-outer VectorOfArray

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Follow-up to #636, which fixed recursivecopy for a VectorOfArray whose outer storage is immutable. With that fix on master the original MWE from SciML/OrdinaryDiffEq.jl#1365 now solves end to end, but nothing in the test suite covers the ODE path — PR #636 only unit-tested recursivecopy. This adds that coverage in test/Downstream/odesolve.jl: an in-place solve of VectorOfArray(SVector{2}([randn(5), randn(5)])) with Tsit5 and Rosenbrock23, checking the return code, that the outer SVector storage is preserved through to sol.u[end], and accuracy against the analytic solution.

Also flips the nested-ArrayPartition Rodas5 @test_broken to @test. That path was fixed upstream in OrdinaryDiffEqRosenbrock v2.6.1, so on current versions the @test_broken reports Unexpected Pass, which errors the testset — test/Downstream is red on master today independent of this PR. Bisected across registered versions locally:

• OrdinaryDiffEqRosenbrock v2.4.2 and v2.6.0: MethodError: Cannot convert an object of type Vector{Float64} to an object of type ArrayPartition{...}
• v2.6.1 and v2.6.3: ReturnCode.Success, deterministic over repeated runs

The compat floor in the Downstream test env is raised to 2.6.1 to match.

Verification

Run locally on Julia 1.11.9 against master + this change, RecursiveArrayTools dev'd, OrdinaryDiffEq v7.2.1, OrdinaryDiffEqRosenbrock v2.6.3:

Test Summary:   | Pass  Total     Time
ODE Solve Tests |   11     11  2m11.9s

Before the change the same file errors on the @test_broken:

ODE Solve Tests: Error During Test at test/Downstream/odesolve.jl:55
 Unexpected Pass
 Expression: (solve(ODEProblem(dyn, ArrayPartition(...), (0.0, 1.0)), Rodas5())).retcode == ReturnCode.Success
Test Summary:   | Pass  Error  Total     Time
ODE Solve Tests |   10      1     11  2m13.4s

The new test also fails as expected against released RecursiveArrayTools v4.3.5 (without PR #636), at recursivecopy in src/utils.jl:38:

setindex!(::SVector{2, Vector{Float64}}, ...) is not defined

Runic formatted.

Links

• error when using VectorOfArray with underlying StaticArray OrdinaryDiffEq.jl#1365
• Fix recursivecopy for VectorOfArray with immutable outer storage #636

🤖 Generated with Claude Code

https://claude.ai/code/session_018DPqBz4Qyy7bZzXZ3sjF45

[ChrisRackauckas-Claude]

CI confirms the @test_broken diagnosis. tests / Downstream on this PR:

job	this PR (#637)	#638 (version-string bump only, i.e. master's behavior)
tests / Downstream (julia 1)	pass	fail
tests / Downstream (julia lts)	pass	fail
tests / Downstream (julia pre)	pass	fail

#638 changes nothing but version = "4.3.5" → "4.3.6", so its red Downstream group is master's current state, and this PR is what makes it green again.

The remaining red checks are identical on both PRs and unrelated to VectorOfArray/recursivecopy — pre-existing breakage from upstream version drift since master last ran CI on 2026-07-24:

• Downgrade (Core) — Unsatisfiable requirements ... no versions left during resolution
• LabelledArrays.jl/Core — FieldError: type StaticArraysCore.SizedArray has no field y
• OrdinaryDiffEq.jl/Downstream (DiffEqBase tests) — ArgumentError: array of type Diagonal{Float64, Vector{Float64}} and size (2, 2) under Rosenbrock23 with sparse AD
• SciMLSensitivity.jl/Core1 — MethodError: Cannot convert Vector{ReverseDiff.TrackedReal{...}} to ReverseDiff.TrackedArray{...}