[ionos-landgraf-vin]

Summary

• `extend "height"` on a stack child now takes the extra height when its column (a layer child, also through a `boundary`) is grown to the tallest sibling; the ranks below move down, so e.g. Building Blocks line up across columns without hand-tuned gaps.
• New container attribute `anchor` (`top|bottom|left|right|center`) sets where content sits when a box grows (default unchanged: top for height, centered for width).
• Docs (`docs/diagram.md`), attribute validation and tests added.

Test plan

• `bundle exec rake` (tests, steep, rubocop)

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