Fix relevant FastPower downstream jobs

[ChrisRackauckas-Claude]

Ignore this PR until reviewed by @ChrisRackauckas.

Summary

• Correct the four OrdinaryDiffEq group names that lost their underscores.
• Select Integrators_I, Integrators_II, Regression_I, and Regression_II instead of silently selecting no matching branch.
• Remove the SimpleDiffEq row because SimpleDiffEq has no FastPower dependency or test reference, so developing the FastPower checkout cannot affect that lane.

The organization-wide audit found that recent green jobs for the old OrdinaryDiffEq names emitted no test summaries. It also found that the SimpleDiffEq lane was structurally irrelevant, even though it could run SimpleDiffEq tests successfully.

Local verification

Each corrected lane was run with this FastPower branch developed into OrdinaryDiffEq:

• GROUP=Integrators_I: passed; final Step Limiter summary 129/129.
• GROUP=Integrators_II: passed after 4,992.6 seconds; final IMEX Split Function summary 15 passed, 1 pre-existing broken.
• GROUP=Regression_I: passed.
• GROUP=Regression_II: passed; IIP/OOP summary 175/175.

The final workflow also passed:

• direct assertion that all four corrected groups exist and the SimpleDiffEq row does not;
• direct recursive scan confirming SimpleDiffEq's package, source, and tests do not reference FastPower;
• actionlint .github/workflows/Downstream.yml;
• julia +1.12 --startup-file=no -m Runic --check .;
• git diff --check.

[ChrisRackauckas-Claude]

All three Core failures are self-hosted runner losses, not test assertions: jobs 87327890031 (LTS), 87327890049 (1), and 87327889995 (pre) ran on separate self-hosted-4vcpu-8gb runners; in every job the main test step is still marked in_progress, all later steps are pending, and no failed-step log exists. They are being added to SciML/.github#116. The four corrected OrdinaryDiffEq downstream groups were each run locally and passed, including Integrators_II after 4,992.6 seconds (15 pass, 1 pre-existing broken).

[ChrisRackauckas-Claude]

CI is now terminal: 19 checks succeeded and the only three failures are the already-classified self-hosted Core runner disconnects (LTS, Julia 1, and pre). All three exact downstream groups also passed locally, including the long Integrators_II run (15 passed, 1 pre-existing broken). No package-test failure remains attributable to this workflow change.

[ChrisRackauckas-Claude]

Added commit 85c04f0 to remove the SimpleDiffEq lane. A recursive scan of SimpleDiffEq's Project.toml, source, and tests found no FastPower declaration or reference, so the lane could only test an unaffected package. The final workflow passes actionlint, Runic, diff checks, and explicit matrix/relevance assertions.