Migrate navigation roots to mergeProps

[joshblack]

Part of the ADR-025 prop merging migration.

This PR updates our page and navigation component families to merge component-authored and consumer-authored props through mergeProps. The migration preserves consumer precedence, class names, refs, handlers, and accessibility behavior while removing 45 findings from the migration report.

Changelog

New

• Add type coverage for attributes controlled by UnderlinePanels.Panel.

Changed

• Update ActionBar, Breadcrumbs, NavList, PageHeader, Pagehead, SideNav, SplitPageLayout, SubNav, TabNav, UnderlineNav, UnderlinePanels, and UnderlineTabbedInterface to use the ADR-025 prop-merging convention.
• Update UnderlinePanels.Panel types to exclude attributes that are always controlled by the component.

Removed

• Remove unsupported aria-labelledby, data-selected, hidden, id, and role attributes from the public UnderlinePanels.Panel TypeScript surface.

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 corrects the public types to match existing runtime behavior. UnderlinePanels already owns each panel's ID, role, visibility, selected state, and accessible relationship.

Testing & Reviewing

• Review consumer precedence and event composition at each migrated root.
• Review the controlled UnderlinePanels.Panel attributes and patch changeset.
• Confirm the mergeProps migration report decreases from 189 findings across 78 files to 144 findings across 66 files.
• Targeted component tests and the @primer/react type-check pass.

[changeset-bot]

🦋 Changeset detected

Latest commit: 057b4de

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.