Canonical CI: grouped-tests.yml + root test/test_groups.toml

[ChrisRackauckas-Claude]

Converts the root Tests.yml test workflow to the canonical thin caller of SciML/.github grouped-tests.yml@v1, with the version/OS matrix declared once in a ROOT test/test_groups.toml.

Change

• .github/workflows/Tests.yml: the hand-maintained version × os matrix job is replaced by the thin caller:

  jobs:
    tests:
      uses: "SciML/.github/.github/workflows/grouped-tests.yml@v1"
      secrets: "inherit"

  Filename and top-level name: "Tests" are preserved so branch-protection status checks stay valid. on: triggers (pull_request/push on master with paths-ignore: docs/**, plus the weekly schedule) and the concurrency: block are kept verbatim. No with: inputs are needed: the package reads the standard GROUP env var, check-bounds default (yes), coverage default (true), and coverage-directories default (src,ext, matching src/ + ext/); there are no apt packages.
• test/test_groups.toml (new): a single capitalized Core group declaring the matrix.

[Core]
versions = ["lts", "1", "pre"]
os = ["ubuntu-latest", "macos-latest", "windows-latest"]

Matrix match: 9/9 exact

The old job ran the full suite (GROUP defaulting to "All") on version ∈ {1, lts, pre} × os ∈ {ubuntu-latest, macos-latest, windows-latest} = 9 cells.

scripts/compute_affected_sublibraries.jl <root> --root-matrix against the new TOML emits exactly 9 (Core, version, runner) cells covering the same {lts,1,pre} × {ubuntu, macos, windows} set. runtests.jl already dispatches the identical testset list for both "All" and "Core" (GROUP == "All" || GROUP == "Core"), so the per-cell behavior is preserved — the only change is the dispatched GROUP string going from the implicit All to Core. The TOML and YAML both parse cleanly.

Category A / QA

This repo already has Core/GPU GROUP dispatch in runtests.jl, and QA (Aqua + ExplicitImports via qa.jl/explicit_imports.jl) runs inline as part of the Core path — there is no separate QA GROUP branch, so no runtests.jl change was made and no separate QA group was added (that would require a source change). Static matrix-verify only; no QA findings.

Untouched: GPU.yml, Downgrade.yml, Documentation.yml, FormatCheck.yml, RunicSuggestions.yml, SpellCheck.yml, TagBot.yml, DocPreviewCleanup.yml, DependabotAutoMerge.yml.

Ignore until reviewed by @ChrisRackauckas.