Labels should be able to flex vertically

A Label doesn't currently do any word wrapping, so its content width is fixed both horizontally and vertically. However, it's only able to flex horizontally, not vertically. I don't see any obvious reason for this.

The issue names no files, tests, or entry points. Begin by locating the Label implementation and its layout tests, then reproduce the current horizontal-only flex behavior. Done means Label content can flex vertically as well as horizontally, with regression coverage.

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