Owner-approved design proposal for #99 (tape-line overlay replaces the axis
rectangle). Wireframe only - no production code.
Settles: the black ~2pt line is a fixed entity in the middle of the 3:4
rectangle; the slab alone pans/zooms/tilts so its long axis lands on the line;
the ticks belong to the line and L = tick to tick; crop % moves only the
overlay's ends ((1 - fill)/2 of the long axis in from the top and the bottom)
and never changes the aspect ratio.
The file header lists the wireframe scaffolding to scrub before this reaches the
production workflow (viewport switcher, coverage board, was/now pair, PROPOSAL
markers, in-stage tutorial labels, sample values, window.__wf debug hook).
