feat(terminal): implement TUI widgets from cbtop spec

Add six new widgets to presentar-terminal:

- Sparkline: compact inline graphs with trend indicators
- Heatmap: 2D grid visualization with color palettes
- Gauge: arc/vertical/compact gauge displays
- Tree: collapsible hierarchical tree view
- ConfusionMatrix: ML classification matrix visualization
- BoxPlot: statistical box-and-whisker plots

All widgets implement Brick + Widget traits with PROBAR-SPEC-009
compliance including assertions, budgets, and verification.

Includes comprehensive test coverage (457 tests pass).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: noahgift

crates/presentar-terminal/src/lib.rs

@@ -43,7 +43,11 @@ pub use error::TuiError;
 pub use input::{InputHandler, KeyBinding};
 
 // Re-export widget types
-pub use widgets::{BrailleGraph, GraphMode, Meter, Table};
+pub use widgets::{
+    BoxPlot, BoxStats, BrailleGraph, ConfusionMatrix, Gauge, GaugeMode, GraphMode, Heatmap,
+    HeatmapCell, HeatmapPalette, MatrixPalette, Meter, NodeId, Normalization, Orientation,
+    Sparkline, Table, Tree, TreeNode, TrendDirection,
+};
 
 // Re-export core types for convenience
 pub use presentar_core::{