Show a visible outline on a selected Divider

[cdinhosbc98-cell]

Fixes #340

Summary

• add the shared focus-ring outline to a selected Divider
• add a leaf-block selected state to the design-system documentation
• document the Divider domain term in CONTEXT.md

Validation

• bun run lint: passed
• Prettier check for all 3 changed files: passed
• git diff --check: passed
• bun run check: the repository-wide format check currently reports pre-existing formatting issues across files outside this change