Centralize CI on SciML reusable workflows

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Converts the remaining inline CI jobs to the centralized SciML/.github reusable workflows pinned at @v1, and adds secrets: "inherit" to every caller. Behavior and matrices are preserved.

Converted (inline -> central caller)

• QA.yml — inline julia-runtest (GROUP=QA, coverage off) -> tests.yml@v1 with group: QA, coverage: false, version matrix ["1"].
• SelfHosted.yml — inline self-hosted bridge tests (GROUP=Bridge) -> tests.yml@v1 with group: Bridge, self-hosted: true.
• FormatCheck.yml — inline fredrikekre/runic-action -> runic.yml@v1.
• SpellCheck.yml — inline crate-ci/typos -> spellcheck.yml@v1.
• Downgrade.yml — inline downgrade (was if: false) -> downgrade.yml@v1 (julia-version: 1.10, skip: Pkg,TOML); kept if: false (disabled per issue Downgrade CI may need adjustment for Optim extension #245).
• Downstream.yml (IntegrationTest) — inline matrix -> matrix of downstream.yml@v1 callers, preserving the exact package+group list (StochasticDiffEq.jl Interface/Interface2/Interface3, SciMLSensitivity.jl SDE1/SDE2/SDE3).

Already central (left as-is)

• Tests.yml — already tests.yml@v1 (secrets: inherit, versions ["1","lts","pre"]).
• Documentation.yml — already documentation.yml@v1.
• TagBot.yml — unchanged.

Other changes

• dependabot.yml — removed the crate-ci/typos version-ignore; pointed the julia ecosystem at dirs that actually contain a Project.toml (/, /docs, /test/qa; dropped the non-manifest /test). github-actions block kept (weekly, /).
• .typos.toml — allowlisted indx, which is a public keyword-argument name in src/pCN.jl (80 occurrences). Not a prose typo; renaming would break the API.

Notes / caveats

• Runic: no formatting changes were needed (repo was already Runic-clean).
• Typos fixed: 0 prose fixes; the only finding (indx) is an identifier and was allowlisted.
• SelfHosted: the original job set JULIA_NUM_THREADS: auto and timeout-minutes: 120; these are not exposed as tests.yml@v1 inputs, so they are dropped. Coverage in the original used fail_ci_if_error: false; the central caller manages coverage itself.
• QA: the central tests.yml@v1 runs julia-buildpkg before tests, which the old inline QA job did not; harmless (QA uses a separate test/qa env).
• Check names will change (e.g. jobs now run as reusable-workflow calls). Branch-protection required-status-check names will need updating after merge.

🤖 Generated with Claude Code