feat: add gradient primitives, GraphNode, and ProgressBarNode

[st0o0]

Summary

• Add Color.Lerp for RGB interpolation and a Gradient record with color stop interpolation, forming the foundation
  for gradient-colored rendering.
• Add GraphNode — a reactive layout node that renders live scrolling graphs in four styles (Blocks, Outline,
  Braille, ASCII) with gradient coloring, configurable data range, and data-driven invalidation.
• Add ProgressBarNode — a single-row progress bar with gradient fill, customizable fill/empty characters, and an
  optional label.
• Add an interactive Graphs & Progress gallery page demonstrating live graph style switching and an animated
  gradient progress bar.
• Add full component documentation for GraphNode and ProgressBarNode. Update color docs with Color.Lerp and
  Gradient sections.

Changes

Color.cs — Add Lerp(Color a, Color b, float t) for linear RGB interpolation.

Gradient.cs — Immutable color gradient with evenly spaced or custom-positioned stops. Sample(float t) returns the
interpolated color.

GraphNode.cs — Reactive layout node with four render styles, gradient support, SetData for immediate invalidation,
and testable TimeProvider-based timer.

GraphStyle.cs — Enum: Blocks, Outline, Braille, Ascii.

ProgressBarNode.cs — Single-row bar with gradient fill, custom chars, label format, and invalidation on value
change. Guarded against double-dispose.

GraphGalleryPage.cs / GraphGalleryViewModel.cs — Interactive demo with live data, style switcher, and animated
progress bar.

docs/components/graph-node.md — Full component documentation with API reference.

docs/components/progress-bar-node.md — Full component documentation with API reference.

docs/styling/colors.md — New Color.Lerp and Gradient sections, updated component table.

docs/components/index.md — Added GraphNode and ProgressBarNode to display components.

Test coverage

• ColorTests — Lerp at boundaries, midpoint, clamp, and non-RGB fallback
• GradientTests — Even distribution, custom stops, boundary sampling
• GraphNodeTests — All four render styles, gradient colors, data-driven invalidation, timer lifecycle
• ProgressBarNodeTests — Half-full rendering, gradient, empty bounds, label, double-dispose safety

Test plan

• dotnet test — all new and existing tests pass
• Run the gallery demo, navigate to Graphs & Progress, switch between all four styles
• Verify gradient colors render correctly on the graph and progress bar
• Verify existing gallery pages are unaffected