Support compiled ODE return codes and specialization

[ChrisRackauckas-Claude]

Draft. Ignore this PR until reviewed by @ChrisRackauckas.

ODESolution.retcode can carry a traced scalar through a compiled solver loop, while the documented ODESolution{T,N} constructor remains available. successful_retcode also accepts scalar carriers convertible to ReturnCode.T.

A SciMLBase-owned ReactantCore extension now reports FullSpecialize for AutoSpecialize ODE functions during compilation. Both instance and type dispatch are covered; ordinary execution and explicit specialization choices retain their behavior. This removes the need for OrdinaryDiffEq to define methods on SciMLBase types. ReactantCore is an optional MIT-licensed dependency.

Verification

Local Julia 1.12.4 Pkg.test() groups:

GROUP=Core
Remake        | 4605   4605  4m17.5s
Testing SciMLBase tests passed

GROUP=Reactant
Reactant specialization | 4  4  5m04.1s
Testing SciMLBase tests passed

GROUP=QA
QA | 123  123  8m15.9s
Testing SciMLBase tests passed

The final QA command was:

GROUP=QA JULIA_PKG_PRECOMPILE_AUTO=0 TMPDIR="$PWD/../OrdinaryDiffEq-shared-reactant/.work/tmp" PIXI_CACHE_DIR="$PWD/../OrdinaryDiffEq-shared-reactant/.work/pixi-base-review" RATTLER_CACHE_DIR="$PWD/../OrdinaryDiffEq-shared-reactant/.work/pixi-base-review" julia +1.12.4 --project=. -e 'using Pkg; Pkg.test(; julia_args=["--compiled-modules=existing"])'

The new specialization regression fails with the unfixed SciMLBase:

Test Failed at test/reactant_specialization.jl:21
Evaluated: Float32[0.0, 0.0] == Float32[1.0, 2.0]

The same test passes with the extension (4/4 above). The docs build, Runic, spelling, and whitespace checks also passed locally. Reactant is installed only for its dedicated test group, leaving the LTS default test dependencies unchanged.

The pre-existing master QA failure was bisected and fixed separately in the prerequisite linked below. Its immediate parent passed 123/123; the introducing commit failed on the stale Markdown @doc_str import; the separate fix passed 123/123 before incorporation here.

Review limits

Appending the return-code parameter is observable to code spelling every structural ODESolution parameter. The documented {T,N} constructor is preserved and tested. GPU execution and downstream package suites were not run; the companion OrdinaryDiffEq change provides integration coverage.

CI follow-up (2026-09-12)

Core (both architectures), QA, and Reactant CI groups passed. SciMLSensitivity Core1 subsequently received signal 11 in Julia 1.13 LLVM code; this downstream failure remains unresolved: https://github.com/SciML/SciMLBase.jl/actions/runs/34686906579/job/103535370853.

Catalyst downstream CI failed during test-environment setup, before its assertions: Permission denied while writing the installed package’s test/extensions/Project.toml. The same failure occurs on the separate Markdown-import cleanup branch. This CI failure remains unresolved; the local Core, Reactant, QA, and docs results above are the verified coverage.

• Catalyst failure: https://github.com/SciML/SciMLBase.jl/actions/runs/34686906401/job/103535364561
• Same setup failure on the separate cleanup: https://github.com/SciML/SciMLBase.jl/actions/runs/34686221580/job/103533552953

Links

• Solver integration: Share ordinary solver paths with Reactant OrdinaryDiffEq.jl#4436
• Separate QA prerequisite: Remove stale Markdown doc-string macro import #1601

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