mecatui: migrate sessions inventory to bounded.List #1895

Scope

Migrate the selectable /sessions inventory to cmd/mecatui/ui/internal/bounded.List.

Current behavior

renderSessionRows owns a numeric cursor and windows sessionsVisibleRows through legacy scrollWindow in cmd/mecatui/ui/sessions_surface.go. Rows wrap after windowing.

Required outcome

• Model rows with stable session IDs and geometry-aware wrapping.
• Preserve tabs, filtering, pagination, selected-session actions, dynamic action hints, and surface-owned headers/footers/hit regions.
• Render standard selectable rows through presentListRow with declared marker cells.
• Add scenario-first acceptance coverage before implementation; do not expand into transcript browsing (tracked separately).

Verification

Run focused sessions/UI and bounded tests, then the project gates required by the approved acceptance plan.