Stabilize inline visualization loading skeleton

[brsbl]

Summary

The compact inline-visualization loading state could be replaced by a full-height iframe, changing the timeline layout after navigation. Render a complete six-region skeleton immediately and reserve the preview height through the loading-to-iframe transition.

The skeleton uses shimmer-only paint animation, avoiding staged layout or entrance animation.

Changed files

• plugins/inline-vis/app.tsx — fixed-height loading container and six-region skeleton.
• plugins/inline-vis/app.test.tsx — focused regression coverage for the skeleton structure, reserved height, and shimmer-only behavior.

Validation

• The new regression test asserts all six regions render immediately, excludes staged/pulse animation classes, and preserves the requested height when the iframe replaces the loader.
• Tests, typecheck, and real-UI responsive/shimmer-cycle results are not asserted here because their originating-thread artifacts were not independently verifiable from this remote branch.

BB-Thread-ID: thr_g3y2kuichb

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