A highlighted edge draws last on both canvases

[WaylandYang]

On the Ontology page, selecting a class lights its edges — and a dimmed edge from
another program still drew over them, cutting the bright lines into segments. The
Graph page does not have this, because it solved the same problem in 2026-08 and
kept the solution to itself.

zIndex cannot fix it. Sigma draws one program per batch, and zIndex only orders
within a batch; across programs the order is the key order of edgeProgramClasses.
So a dimmed straight edge is still painted after a highlighted curved one.

The fix is to register each edge program a second time under a later key and move
highlighted edges into that batch. That trick now lives in graphCanvas.ts, the
module both canvases already share, instead of in either page:

• withTopLayer(programs) returns the programs plus a …Top duplicate of each.
• drawLast(type) is what a reducer assigns when it highlights an edge.

Both pages use it. The Graph page's hand-written version (curved / lineTop /
curvedTop, with a ternary in the reducer) is replaced by the shared one — same
programs, same key order, no behaviour change there.

Checked by hand on a seven-class schema with nine relations: the selected class's
edges stay unbroken where they cross dimmed ones, and the Graph page renders as
before.

🤖 Generated with Claude Code