Add new commonly needed layouts

- [ ] layout that supports fixed num of columns and uses percentages to set widths with variable height rows
- [ ] layout that supports fixed num of columns and fixed height

The issue names two desired layout behaviors but does not identify files, tests, or entry points. Start by examining the repository's existing layout implementations and tests, then determine the extension points and acceptance criteria for fixed columns, percentage widths, variable-height rows, and fixed-height rows. Done should include agreed behavior and coverage for both layouts.

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