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

[ChrisRackauckas-Claude]

Centralizes this monorepo's GitHub Actions onto the shared SciML/.github reusable workflows (@v1), preserving the repo's existing config (version matrix, default branch master, triggers, paths-ignore, concurrency) and dropping no CI coverage.

Converted / consolidated workflows

File	Before	After
CI.yml (Tests)	tests.yml@v1 with a manual project: matrix listing . + all 7 lib/* sublibraries, over version ["1","lts"]	clean main-package caller of tests.yml@v1 over version ["1","lts"] (project . only)
SublibraryCI.yml	(new)	caller of sublibrary-tests.yml@v1 — auto-discovers lib/*, replacing the per-sublibrary project: entries that used to live in CI.yml
FormatCheck.yml	bespoke fredrikekre/runic-action@v1	runic.yml@v1
SpellCheck.yml	bespoke crate-ci/typos	spellcheck.yml@v1

Sublibrary handling (monorepo)

This repo is a real monorepo: each of the 7 lib/<name>/ dirs has its own Project.toml and test/runtests.jl. The old CI.yml tested them by enumerating them in a project: matrix on tests.yml. That per-sublibrary CI is now consolidated into a single SublibraryCI.yml calling sublibrary-tests.yml@v1, which auto-discovers lib/* and runs only the sublibraries affected by a change. CI.yml is now reserved for the top-level DiffEqProblemLibrary package itself.

Note: sublibrary-tests.yml@v1 reads optional per-sublibrary config from lib/<name>/test/test_groups.toml. None exist here, so the reusable's defaults apply. The draft PR's own CI run will exercise this.

Intentionally left bespoke / untouched

• Downgrade.yml — already a thin caller of downgrade.yml@v1 (julia 1.10, skip Pkg,TOML). Already reusable, so untouched. No DowngradeSublibraries.yml existed, so none was created (no existing sublibrary-downgrade coverage to preserve).
• TagBot.yml — non-CI release automation (JuliaRegistries/TagBot); no reusable equivalent. Left local.

NOTE: required-status-check names change

Routing through the reusables changes the job names that appear as status checks, so branch protection on master must be updated:

• Tests job names now come from tests.yml@v1 / sublibrary-tests.yml@v1 reusable jobs.
• Format and spell-check job names now come from runic.yml@v1 / spellcheck.yml@v1.

Update the required-status-check list accordingly after merge, or required checks will appear "expected" forever.

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Please ignore until reviewed by @ChrisRackauckas.