Migrate Button roots to mergeProps

[joshblack]

Closes #

Migrates Button, IconButton, and LinkButton roots to the ADR-025 prop-merging convention without changing existing prop behavior.

Changelog

New

None.

Changed

None; this is a behavior-preserving refactor.

Removed

None.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

This migration preserves the existing public contract and does not require release notes.

Testing & Reviewing

Review inline class-name composition, ref ordering, and polymorphic consumer prop precedence.

[changeset-bot]

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Latest commit: 2a7e15c

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[github-actions]

⚠️ Action required

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