[Feature] Panel/container widget

[vmarcella]

Overview

Add a panel widget that provides a visual container for grouping other widgets
with optional background and border.

Current State

No response

Scope

Goals:

• Container for child widgets
• Optional background color
• Optional border
• Corner radius option

Non-Goals:

• Complex borders
• Shadows
• Scrolling

Proposed API

pub struct Panel {
  children: Vec<Box<dyn Widget>>,
  background: Option<[f32; 4]>,
  border_color: Option<[f32; 4]>,
  border_width: f32,
  corner_radius: f32,
}

impl Panel {
  pub fn new() -> Self;
  pub fn with_background(self, color: [f32; 4]) -> Self;
  pub fn with_border(self, color: [f32; 4], width: f32) -> Self;
  pub fn with_corner_radius(self, radius: f32) -> Self;
  pub fn add_child(&mut self, widget: impl Widget + 'static);
}

impl Widget for Panel {
  fn render(&self, ctx: &mut UiRenderContext) -> Vec<RenderCommand>;
  fn hit_test(&self, x: f32, y: f32) -> bool;
}

Acceptance Criteria

• Panel renders background color
• Border renders with specified width/color
• Corner radius rounds corners
• Children render within panel bounds
• Hit test considers panel bounds

Affected Crates

lambda-rs

Notes

• Rounded rectangles need custom shader or geometry
• Consider 9-slice for complex backgrounds