[Feature] UI widget base trait and rendering

[vmarcella]

Overview

Define the base widget trait that all UI elements implement and establish the
rendering model for UI components.

Current State

No response

Scope

Goals:

• Widget trait defining common interface
• Widget bounds and layout rect
• Basic rendering abstraction
• Hit testing for input handling

Non-Goals:

• Specific widget types
• Layout system
• Styling

Proposed API

// crates/lambda-rs/src/ui/widget.rs
pub struct Rect {
  pub x: f32,
  pub y: f32,
  pub width: f32,
  pub height: f32,
}

impl Rect {
  pub fn contains(&self, x: f32, y: f32) -> bool;
}

pub trait Widget {
  fn bounds(&self) -> Rect;
  fn set_bounds(&mut self, rect: Rect);
  fn render(&self, ctx: &mut UiRenderContext) -> Vec<RenderCommand>;
  fn hit_test(&self, x: f32, y: f32) -> bool;
}

pub struct UiRenderContext<'a> {
  render_context: &'a mut RenderContext,
  // UI-specific state
}

Acceptance Criteria

• Widget trait is object-safe (can use dyn Widget)
• Bounds stored and queryable
• Hit test uses bounds by default
• Render returns commands for drawing
• Trait extensible for specific widgets

Affected Crates

lambda-rs

Notes

• Consider immediate-mode alternative (egui integration)