mecatui: bound saved-memory inventory and detail browser #1896

Scope

Migrate the saved-memory inventory and its read-only detail/history browser in cmd/mecatui/ui/usermodel.go to bounded controls.

Current behavior

The inventory has a logical cursor and independently centers wrapped rendered lines with windowRenderedLines. Detail/history has a physical scroll offset and the same helper.

Required outcome

• Use bounded.List for the selectable inventory with stable memory-entry IDs and reveal behavior for wrapped descriptions.
• Use bounded.Viewport for read-only detail/history physical-line browsing.
• Preserve Enter-to-detail, back navigation, state/reset behavior, fixed footer geometry, and surface-owned chrome.
• Use presentListRow for the inventory's standard selectable rows.

Verification

Add scenario-first acceptance coverage and run focused user-model/UI and bounded tests before project gates.