feat: add gradient labels and line labels to GPU comparison charts

Extends the gradient label feature (parallelism strategy coloring on
roofline paths) and line labels (hardware name pills) from ScatterGraph
to GPUGraph, so date-range comparison charts get the same visual tools.

Key changes:
- Convert GPUGraph roofline layer from declarative to custom D3 rendering
- Add gradient stops, parallelism pill labels, and line labels with
  collision avoidance
- Add Gradient Labels, Line Labels toggle switches to the legend sidebar
- Wire up gradient nudge toast for the comparison chart
- Color scatter points by parallelism strategy when gradient labels are on
- Handle zoom updates for all label types

Author: functionstackx