perf(erd): hover rebuilds and repaints the whole canvas; no viewport culling; first layout on the UI thread #1169

Problem

• Hover rebuilds the whole canvas. onHover calls setState on ErdView: every _TableCard (≈ 3 000 text rows for 200 tables × 15 columns) is rebuilt and every edge repainted; roundedPath is rebuilt for each route on every paint.
• _isFocused scans all relations for each card: O(tables × relations) per build.
• One RepaintBoundary around the whole canvas: any change repaints all cards and edges.
• No viewport culling: InteractiveViewer(constrained: false) builds and lays out every card, also the ones off screen.
• First layout and routing run on the UI thread inside _load (ErdLayout.compute + ErdRouter.route), a hitch when the Diagram tab opens on a large schema.

Scope

• Hover / drag / selection state in ValueNotifiers; cards listen only to whether they are highlighted.
• RepaintBoundary per card; edges in two painters (base layer, highlighted layer); Paths cached when routes change.
• Adjacency map computed once per schema.
• Build only cards intersecting the viewport (+ margin) via InteractiveViewer.builder.
• ErdLayout.compute + first route() in Isolate.run (schema and offsets are plain data), spinner until done.
• ERD scenario in benchmark/app_perf_bench.dart: open / hover / drag on a generated 200-table schema.

Acceptance

• Hover over a card rebuilds only the affected cards (widget test counting builds).
• Benchmark: hover and drag frames within the 120 Hz budget on 200 tables; opening the diagram has no frame over 16 ms on the UI thread.