mecatui: evaluate bounded.Viewport for stored-session transcript #1903

Scope

Evaluate the stored-session transcript inspector in cmd/mecatui/ui/sessions_surface.go for bounded.Viewport adoption.

Current behavior

The static replay inspector uses Bubble Tea viewport.Model, conversation-rendered lines, tail-stuck state, standard viewport navigation, and a transcript footer. It is distinct from the active conversation viewport.

Required outcome

• Establish that a bounded physical-line viewport can preserve replay loading, dynamic rendered lines, AtBottom/transcriptStuck behavior, navigation, wheel behavior, and the footer.
• Keep the active live conversation viewport, text selection, streaming-tail behavior, and semantic reading anchors out of scope.
• Explicitly retain the inspector's current viewport if the static/replay contract cannot be preserved cleanly.

Verification

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