fix(mosaic): keep Flow transitioning for the root height transition only

[alexcarpenter]

Description

Stacked on #10152.

Flow.Root's data-transitioning now lasts from a step change until the root's own height transition finishes. Before, it was built from the exiting steps plus a settle wait. Step enter/exit durations and the height duration are now independent: either can be longer without the height snapping at the end.

• Flow.Step no longer reports exits to the root, so the exiting-step tracking and the settle state are removed.
• useAnimationsFinished takes an optional filter. Flow waits only on height transitions, so other animations on the root (including infinite ones) can no longer leave it stuck in data-transitioning. Other callers don't pass a filter and are unchanged.
• If the height does not transition (equal heights, reduced motion), the attribute clears at once.
• The Flow README now describes data-transitioning and data-height-change.

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

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Latest commit: bcf274f

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