Migrate CI to centralized SciML/.github reusable workflows

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Normalizes CI to the SciML centralized reusable workflows (all pinned @v1, each caller passes secrets: "inherit").

Workflows converted

• Tests.yml (name: CI) → calls tests.yml@v1. Preserves the exact matrix group: [CPU, QA] × version: [lts, "1"], fail-fast: false, the push/pull_request triggers with paths-ignore: docs/**, and the concurrency group. Coverage left at the workflow default (true) to match the original (it collected coverage via codecov).
• Downgrade.yml → calls downgrade.yml@v1 with julia-version: "1.10" and skip: "Pkg,TOML". Kept disabled with if: false (and the original explanatory comment referencing Re-enable downgrade CI tests #193), exactly as before.
• FormatCheck.yml (name: format-check) → calls runic.yml@v1. Same triggers as before.
• SpellCheck.yml (name: Spell Check) → calls spellcheck.yml@v1.

Workflows added

• documentation.yml → calls documentation.yml@v1 (repo has docs/make.jl and docs/Project.toml). This is the standard centralized Documentation build/deploy caller. Note the existing GPU.yml already builds docs on self-hosted GPU runners; that workflow is left untouched.

Workflows left unchanged

• GPU.yml — self-hosted CUDA tests + GPU docs build; outside the standard set, preserved as-is.
• TagBot.yml — unchanged.

Dependabot

• Removed the crate-ci/typos ignore block from the github-actions updates (standing policy: keep everything current, no per-dependency ignores).
• Preserved the existing julia ecosystem block (directories /, /docs, /test; daily; all-julia-packages group). Not narrowed.

CompatHelper

• No CompatHelper.yml was present; nothing removed.

Runic formatting

• The repo already ran Runic in CI. Ran Runic.main(["--check", ...]) locally (Runic v1.7.0): clean, exit 0. No formatting changes were needed or made.

Typos

• Ran typos locally against the existing .typos.toml: clean, exit 0. The centralized check uses a newer typos version (v1.47.0 vs the previous pinned v1.18.1); no new findings observed locally.

Heads-up

• Check names change. Branch-protection required-status-checks will need to be updated to the new job names (e.g. Tests, Documentation, Runic Format Check, Spell Check with Typos).

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