Describe your changes

Added notes to openspec

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Summary by cubic

Reorganized the morphism module and preserved public re-exports, added Algebraic Causaloid design notes to openspec, and bumped CI to actions/checkout@v7.

• Refactors

  • Moved Morphism and FnMorphism to morphism/morphism_base.rs; kept morphism/mod.rs as a thin module.
  • Namespaced Endomorphism under morphism and re-exported Morphism, FnMorphism, and Endomorphism from lib.rs.
  • Updated test imports to the new paths.
  • No behavior changes; API remains stable via re-exports.

• Dependencies

  • Upgraded actions/checkout to v7 across all GitHub workflows.

^{Written for commit 439a54d. Summary will update on new commits.}