mecatui: evaluate bounded.Viewport for Dream review #1901

Scope

Evaluate and, if justified, migrate Dream plan/receipt review in cmd/mecatui/ui/dream.go to bounded.Viewport.

Current behavior

Dream uses an increment/decrement scroll offset and a render-time clipped slice. It is a multi-state action surface: apply, dismiss, and regenerate actions coexist with reader navigation.

Required outcome

• Classify whether the reader portion is a safe bounded.Viewport candidate before implementation.
• Preserve action-key routing, card/footer geometry, multi-state transitions, and current offset clamping/reset semantics.
• Explicitly retain the local window if the action surface coupling makes migration inappropriate.

Verification

Add scenario-first acceptance coverage for the approved decision and run focused Dream/UI and bounded tests before project gates.