Fix infinite recomposition in flexible containers

[dfabulich]

Flexible fill could cause contentFlexibleWidthMax/contentFlexibleHeightMax to increase due to a side effect, causing a decrease in the next side effect, causing an increase in the next side effect, and so on.

Now, we only allow monotonically increasing values.

Fixes #389

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

Cursor generated the merge function. (I'm not entirely sure what the right expansion rank ought to be; it doesn't match the ordering in StackLayouts.kt.)

I visually inspected the Stacks playground, verified that infinite recomposition halted in automated tests. skiptools/skipapp-showcase#83