perf(erd): eliminate per-frame allocations of Path, Paint, and TextPainter in ErdRelationPainter #1518

Summary

In lib/features/erd/widgets/erd_relation_painter.dart (lines 36–98):

Inside ErdRelationPainter.paint(Canvas canvas, Size size), the rendering loop iterates over all diagram routes on every single animation frame of panning, zooming, or dragging:

for (final r in routes) {
  if (_focused(r) != pass || r.points.length < 2) continue;
  final paint = material.Paint() ... // Reallocated for each route
  canvas.drawPath(roundedPath(r.points), paint); // Full Path recalculated for each route

  // Cardinality labels '1' and '*':
  _label(canvas, r.relation.oneToOne ? '1' : '*', ...);
}

Inside _label():

final tp = material.TextPainter(
  text: material.TextSpan(text: text, style: TextStyle(...)), // 3 allocations per label
)..layout(); // Synchronous text layout pass
tp.paint(canvas, ...);
tp.dispose();

On a realistic schema with 150 tables and 300 relations:

• 00 \text{ relations} \times 2 \text{ labels} = 600 \text{ TextPainter} + 600 \text{ TextSpan} + 600 \text{ TextStyle} + 300 \text{ Paint} + 300 \text{ Path}$
• ~2,400 heap allocations PER FRAME
• At 120 FPS, this generates ~290,000 allocations per second, triggering constant GC pressure and dropping frame rates to 40–50 FPS during pan/zoom.

Architectural Solution: Zero-Allocation CustomPainter

1. Path Caching in ErdRoute:
   • Precompute and store Path? _cachedPath on ErdRoute when layout/routing completes.
   • Draw the precomputed path directly without calling roundedPath(r.points) per frame.
2. Reused Static Paint Instances:
   • Maintain reusable _basePaint and _highlightPaint instances, modifying colors and stroke widths in place instead of creating Paint() in a loop.
3. Pre-laid-out Cardinality Painters / Picture Caching:
   • The cardinality markers are always '1' and '*'. Pre-layout them once in static TextPainter instances or pre-record their Picture.

Acceptance Criteria

• Panning and zooming on schemas with 300+ relations performs zero TextPainter, Paint, or Path allocations per frame.
• ERD diagram maintains steady 120 Hz frame budget during pan/zoom.