#21419 feat(cmux-next): animate New Tab search and Ask switching

What changed

Animate the existing New Tab kind cycle when Tab/Shift+Tab switches between browser/search and agent/ask. The field remains one stable composer while the context chip and placeholder hand off directionally, matching the browser-style transition.

Scope

This extracts only the kind-switch animation and deterministic gallery coverage from the broader composer/gallery work in #21200. The underlying Tab cycle already exists on feat-cmux-next.

Validation

• Gallery fixture coverage carried over from the canonical New Tab entry
• Portable webview checks to run in CI
• No native build on the MacBook Air

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Summary by cubic

Animates New Tab kind switches so Tab/Shift+Tab transitions between browser/search and agent/ask reuse the composer field, while the context chip and placeholder swipe directionally; consecutive switches restart the animation.

• listMotion now honors the agentMotion policy boundary when a list update races the theme transition, routes the check through a shared helper, and stops measuring on every render for callers that allocate fresh key arrays.
• The apps gallery flow test now waits for the rendered empty states before asserting their text.