Re-enable the Downstream adjoint test

[ChrisRackauckas-Claude]

Summary

SciML/SciMLBase.jl#1297 has merged and registered SciMLBase 3.x declares RecursiveArrayTools = "4", so the TODO that kept Downstream/adjoints.jl commented out is satisfied. This restores the Downstream Adjoint safetestset and puts back the ForwardDiff / SciMLBase entries in test/Downstream/Project.toml that were removed while waiting on that compat bump.

Verification

export TMPDIR=/home/crackauc/sandbox/goals/qa-hygiene/jobs/rat-tests/tmp
export JULIA_DEPOT_PATH=/home/crackauc/sandbox/goals/qa-hygiene/jobs/rat-tests/scratch/julia_depot
GROUP=Downstream julia --project -e 'using Pkg; Pkg.test()'

Observed (excerpt):

Test Summary:            | Pass  Total     Time
Downstream Adjoint Tests |    1      1  2m24.7s
...
Test Summary:              | Pass  Total     Time
DiffEqArray Indexing Tests |  358    358  8m13.5s
     Testing RecursiveArrayTools tests passed
EXIT=0

Also confirmed: gh pr view 1297 -R SciML/SciMLBase.jl → MERGED; SciMLBase Project.toml / registry Compat has RecursiveArrayTools = "4" under [3].

Not verified

• Other CI groups (Core, GPU, AD, QA, NoPre) were not re-run for this change.
• LTS / Julia 1.x matrix beyond the local Julia 1.12.4 Downstream run.

Please ignore this draft until reviewed by @ChrisRackauckas.

Risk assessment

• Risk: Low — restores a previously intentional Downstream test once its upstream blocker cleared; no production code changes.
• Blast radius: Downstream CI group only (test/runtests.jl Downstream body + Downstream env deps).
• Evidence: Full GROUP=Downstream Pkg.test() passed locally including the re-enabled adjoint safetestset.
• Independent review: Claude Code (Opus 5.5 subagent) rated it low, high confidence: test-only change; reran GROUP=Downstream on the PR head (Downstream Adjoint Tests 1/1, DiffEqArray Indexing 358/358, tests passed); added deps ForwardDiff and SciMLBase are MIT. VERDICT MERGE.
• Merge: needs review

Harness: Cursor Agent CLI 2026.09.18-9a7762b; model: auto; transcript: /home/crackauc/sandbox/goals/qa-hygiene/jobs/rat-tests/log.txt on amdci2

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