Restrict the docs CairoMakie compat bound to 0.15

[michakraus]

What changes

[compat] CairoMakie in the documentation environment is "0.15", narrowed from
"0.12, 0.13, 0.14, 0.15".

Only 0.15 is tested, and CairoMakie 0.12 fails to precompile on Julia 1.13, so the wider range
claimed support that does not hold. The package itself does not depend on CairoMakie; only its
documentation build does.

The same restriction is applied in this tree to GeometricBrackets' [compat] and to VlasovMethods'
Makie extension, which carried the identical range.

Why

A dependency's [compat] bound is a claim the CI matrix should be able to keep, and only the
versions CI runs are tested. The author restricted CairoMakie to "0.15" everywhere on 2026-10-12.

Evidence

• docs/Project.toml is the only changed dependency file; no Julia source changes.
• The docs environment resolves on Julia 1.13.1 to CairoMakie 0.15.15, Documenter 1.19.0 and the
  working-tree SimpleSplines, and the resolution leaves docs/Project.toml byte-for-byte unchanged.
• Documenter.doctest over docs/src/ passes with no failures, and the tutorial's first spline plot
  renders to PNG under CairoMakie 0.15.15.
• The full suite is green on the branch: 69 s.

Pre-PR verification

• Fixed: none needed.
• Unresolved: none; no diff-attributable findings.
• Pre-existing: no adjacent findings. The repository's recorded JET/Revise issue is outside this
  diff and was not re-investigated.
• Checked and clean: the CairoMakie bound is exactly "0.15", it excludes the removed minor
  versions, it stays docs-only, and it resolves as above. No stale comment or docstring is
  introduced, and the rationale is confined to the new [Unreleased]/Changed changelog entry.
• Not checked: whole-package dependency-duplication analysis, and the piracy, inference and
  allocation points, which do not apply to a compat line. The full documentation build and the Julia
  1.11 floor were not exercised here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.13%. Comparing base (a3a5e43) to head (2eb4c4e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   91.13%   91.13%           
=======================================
  Files           9        9           
  Lines        1162     1162           
=======================================
  Hits         1059     1059           
  Misses        103      103           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.