mecatui: evaluate bounded viewports in Approval review #1902

Scope

Evaluate the Approval surface's three independently scrollable review regions in cmd/mecatui/ui/approval_surface.go for bounded viewport adoption: full plan, full arguments/details, and in-card arguments.

Current behavior

The full plan and arguments views use Bubble Tea viewport.Model; the in-card body uses a manual offset over a layout-derived line range. Pinned verdict controls and modal wheel capture are part of the interaction contract.

Required outcome

• Decide whether bounded.Viewport can serve each region without weakening the modal/layout contract; retain any region that cannot.
• Preserve approval queue and fingerprint invalidation, raw/pretty toggle state, width/height reinitialization, pinned action-bar geometry, and wheel capture.
• Keep card-layout/hit-region coupling exact for the in-card body.

Verification

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