docs(readme): regenerate pipeline figure in dj.Diagram notation (adaptive)

Redraw the README example-pipeline figure in the modernized dj.Diagram notation
(muted tier palette + colored strokes, Imported/Computed ovals, Manual/Lookup
rectangles, thick/thin cardinality edges, master-part entity cluster). Activity
is now a Part of Segmentation: cell_mask and trace live on Segmentation.Activity
and make() inserts into the part. The SVG is theme-adaptive (auto) — light by
default, dark via prefers-color-scheme — so reference the SVG directly and drop
the stale raster PNG.

Author: dimitri-yatsenko