mecatui: evaluate bounded.Viewport for Help browser #1900

Scope

Evaluate and, if justified, migrate the Help overlay's read-only body in cmd/mecatui/ui/help.go to bounded.Viewport.

Current behavior

The root owns helpScroll and uses windowRenderedLinesWithIndicator with height-derived fitting, custom overflow text containing live bindings, and tiny-height behavior.

Required outcome

• Establish a concrete inconsistency or needed feature before changing this deliberately surface-specific browser.
• If migrating, retain live-binding overflow text, tiny-height handling, keyboard behavior, and surface-owned indicators/chrome.
• Explicitly retain the current implementation if the primitive supplies no net behavior or correctness improvement.

Verification

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