perf(erd): relation painter reuses its Paint, rounded paths and cardinality labels (#1518)

[ZhuchkaTriplesix]

Closes #1518.

ErdRelationPainter.paint allocated, for every route and every repaint, a Paint, a rounded Path and two laid-out TextPainters (with a TextSpan and a TextStyle each).

• One Paint per paint() call; its colour and width are set once per pass (others / focused) instead of per route.
• Rounded paths are cached per ErdRoute in an Expando: a repaint that only changes the focused table or the picked relation (hover, click) keeps the same routes and no longer rebuilds 300 paths.
• Cardinality labels are only ever 1 and * in at most three colours, so each is laid out once and shared (static cache of 16 entries, disposed and restarted when a theme change brings new colours).

Not measured: pan and zoom do not repaint this painter at all (the InteractiveViewer transforms a layer), so the gain is on focus / hover / pick repaints and table drags, where the router's A* dominates. The erd_open_200 bench scenario does not run yet (#1546), so CI cannot show a number for this. Tests pin that every state (nothing focused, focused table, picked relation, route with one point, more colours than the cache holds) still paints, and shouldRepaint.