ci: route CI through SciML/.github reusable workflows (@v1)

[ChrisRackauckas-Claude]

Centralize CI through SciML/.github reusable workflows (@v1)

This routes the repo's bespoke standard GitHub Actions workflows through the shared reusable workflows in SciML/.github/.github/workflows/*.yml@v1, preserving each workflow's existing matrices, triggers, and config. Pure YAML/git rewiring — no source changes.

Converted workflows

File	Now calls	Preserved config
Downgrade.yml	downgrade.yml@v1	group: Core, julia-version: 1.10, skip: Pkg,TOML, allow-reresolve: false; master push/PR triggers + paths-ignore: docs/**
Downstream.yml	downstream.yml@v1	Full SciML downstream package/group matrix (SciMLBase, DiffEqBase x3, DiffEqSensitivity, OrdinaryDiffEq x2, DelayDiffEq, SciMLSensitivity Core1-6, LabelledArrays); julia-version: 1; IntegrationTest name + triggers
FormatCheck.yml	runic.yml@v1	Runic format check; master/main/release- push + tags + PR triggers
SpellCheck.yml	spellcheck.yml@v1	pull_request trigger

Intentionally left bespoke (coverage cannot be faithfully expressed by a reusable)

• Tests.yml — sets JULIA_NUM_THREADS: "2" on the runtest step to exercise the threaded FastBroadcast / VectorOfArray{SArray} regression tests (issues Multithreading with RecursiveArrayTools.VectorOfArray is broken #564 / Multi-threading VectorOfArray of SVectors is broken on RecursiveArrayTools.jl version 4 #570) in test/interface_tests.jl. The reusable tests.yml@v1 has no thread input, so converting it would silently run those tests single-threaded and drop the regression coverage. Per the centralization spec ("never lose coverage"), this workflow is left as-is.
• GPU.yml — targets a specific [self-hosted, Linux, X64, gpu] runner with GROUP=GPU. The reusable self-hosted input resolves to a bare self-hosted label and cannot target the GPU-labeled runner, so this is left local.
• Documentation.yml — already a thin caller of documentation.yml@v1; untouched.
• TagBot.yml — non-CI release automation (JuliaRegistries/TagBot); no reusable equivalent, left local.

Monorepo / sublibrary handling

The repo has 3 sublibraries under lib/ (RecursiveArrayToolsArrayPartitionAnyAll, RecursiveArrayToolsRaggedArrays, RecursiveArrayToolsShorthandConstructors), each with its own Project.toml + test/. However, there is no per-sublibrary CI (no CI_*.yml, no SublibraryCI.yml, no DowngradeSublibraries.yml). The sublibraries are tested as GROUPs of the main package's single Tests.yml (RaggedArrays, ArrayPartitionAnyAll, ShorthandConstructors), and none of them carry the test/test_groups.toml that sublibrary-tests.yml@v1 auto-discovery requires. Introducing a SublibraryCI.yml here would discover nothing and would not replace the existing GROUP-based coverage, so no sublibrary CI workflow was added (N/A). Since Tests.yml is left bespoke for the threading reason above, its sublibrary GROUP coverage is preserved unchanged.

NOTE: required-status-check names change

Converting to reusable workflows changes the job names that appear as status checks (e.g. the reusable emits Tests / Downstream Tests - ..., Downgrade, Runic, Spell Check with Typos). Branch protection required-status-check names must be updated accordingly, or required checks will report as missing.

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Please ignore until reviewed by @ChrisRackauckas.