Simplify root CI to grouped-tests.yml (matrix from test/test_groups.toml)

[ChrisRackauckas-Claude]

Summary

Converts the root test workflow (.github/workflows/CI.yml, status check name: "Tests") from a hand-maintained group × version matrix that directly called tests.yml@v1 into a thin caller of the new grouped-tests.yml@v1. The matrix now lives once in a new root test/test_groups.toml.

SublibraryCI.yml (the sublibrary-project-tests.yml@v1 project-model workflow) is intentionally left untouched — this PR only touches the root umbrella package's own test groups.

.github/workflows/CI.yml (in place)

on: triggers, concurrency:, and the workflow name: "Tests" are preserved verbatim, so the branch-protection status-check name is unchanged. The job body becomes:

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

No with: inputs are needed: the old job used tests.yml@v1 defaults (check-bounds: yes, coverage: true, single ubuntu-latest, default threads, no continue-on-error) and the root test/runtests.jl dispatches on the default GROUP env var (get(ENV, "GROUP", "All")).

New test/test_groups.toml

[Core]
versions = ["lts", "1", "pre"]

[QA]
versions = ["lts", "1"]

This reproduces the old matrix exactly (the old workflow ran group: [Core, QA] × version: [lts, 1, pre] with exclude: QA/pre).

Verified matrix match

Ran scripts/compute_affected_sublibraries.jl <repo> --root-matrix from SciML/.github@v1 against the new test/test_groups.toml and compared its (group, version) set against the set the old CI.yml matrix produced:

OLD: [(Core,lts), (Core,1), (Core,pre), (QA,lts), (QA,1)]
NEW: [(Core,lts), (Core,1), (Core,pre), (QA,lts), (QA,1)]
missing: []   extra: []   EXACT MATCH: 5/5

TOML and both workflow YAML files parse cleanly. The package test suite was not run locally (heavy; CI validates).

---

Ignore until reviewed by @ChrisRackauckas.