Modernize CI setup

[devmotion]

Brings the CI setup in line with current Julia ecosystem practice.

CI changes

• Dependabot replaces CompatHelper: adds a julia ecosystem entry for / and /docs, grouping package updates into one weekly PR. CompatHelper.yml is removed.
• Spell check: new SpellCheck.yml workflow running crate-ci/typos.
• Test matrix: lts and 1 on Linux, macOS and Windows with arch: default. On macos-latest (Apple Silicon) this runs native aarch64 binaries. The 32-bit x86 job is unchanged. pre now runs only on Linux and is allowed to fail. The separate JuliaNightly.yml workflow is removed.
• Docs: build on Julia 1 and install dependencies with Pkg.instantiate(), relying on [sources] in docs/Project.toml instead of Pkg.develop. Adds actions: write so julia-actions/cache can prune old caches.
• Downgrade: run on lts, actually pass mode to julia-downgrade-compat, and add the cache, permissions and concurrency settings. Removes the unused group/GROUP matrix entry, a stale comment and the no-op skip: Pkg,TOML. In v2 the action leaves stdlibs out of the downgrade on its own.
• IntegrationTest: add permissions so the cache works, and drop the redundant arch: x64. Drop Diffractor: it only ran on Julia nightly, where its dependency Cthulhu doesn't precompile, and it hasn't had a code change since June 2025. The ChainRulesOverloadGeneration job stays; its failing demo tests are fixed in Fix type piracy in demo tests ChainRulesOverloadGeneration.jl#22.

Typo fixes

Fixes about 40 misspellings found by typos in comments, docstrings, docs and tests, including the test-only function mixed_vararg_type_constaint. Two variable names that typos flags are renamed instead of allowlisted: dne → nt in test/tangent_types/abstract_zero.jl, and somes/some → results/result in a docs example.

Repo settings needed

• Add CODECOV_TOKEN as a Dependabot secret (Settings → Secrets and variables → Dependabot). Dependabot PRs can't read normal Actions secrets, and fail_ci_if_error: true would otherwise fail CI on them.
• Update the required status checks: remove the pre matrix jobs and JuliaNightly, and add Spell Check.

Testing

• typos passes locally on the whole repo.
• Pkg.test() passes locally on Julia 1.10.12 (lts) and 1.13.1.
• The workflow files parse as valid YAML. actionlint was not run.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.39%. Comparing base (0aa1554) to head (4e89211).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #715   +/-   ##
=======================================
  Coverage   93.39%   93.39%           
=======================================
  Files          15       15           
  Lines         999      999           
=======================================
  Hits          933      933           
  Misses         66       66           

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