desktop: cache assembled workspace vertex buffer at idle

Workspace mode rebuilt ~100k primitive vertices every redraw (8.7ms CPU
in debug, dominating the 120fps frame budget) even when nothing changed,
unlike single-session which caches its primitive vertex buffer.

Add a workspace-level primitive vertex cache keyed on the settled
content/layout state, consulted only when no workspace animation is
active, so idle redraws reuse the previously assembled buffer.