desktop: reuse workspace text pane apps on cache hits

Avoid building SingleSessionApp for cached workspace session panes and benchmark navigation vertices through the real pane cache path. Scroll bench (60 frames): workspace_navigation_subphases.vertices 434ms -> 0.204ms; workspace_full_frame_subphases.text_panes 109ms -> 8.304ms; workspace_full_frame_subphases.vertices 0.183ms. Validated cargo check/build/test.