[Feature] UI event propagation and focus

[vmarcella]

Overview

Add event propagation system for UI that handles focus, hover tracking, and
input event routing to the correct widget.

Current State

No response

Scope

Goals:

• Focus management (which widget receives keyboard input)
• Hover tracking for mouse enter/leave
• Event bubbling (child to parent)
• Event capture (parent before child)

Non-Goals:

• Custom event types
• Event cancellation

Proposed API

pub struct UiContext {
  root: Box<dyn Widget>,
  focused: Option<WidgetId>,
  hovered: Option<WidgetId>,
}

impl UiContext {
  pub fn new(root: impl Widget + 'static) -> Self;
  pub fn handle_mouse_move(&mut self, x: f32, y: f32);
  pub fn handle_mouse_button(&mut self, button: Button, pressed: bool);
  pub fn handle_key(&mut self, key: Key);
  pub fn set_focus(&mut self, widget: WidgetId);
  pub fn focused(&self) -> Option<WidgetId>;
  pub fn render(&mut self, ctx: &mut RenderContext) -> Vec<RenderCommand>;
}

Acceptance Criteria

• Click sets focus to clicked widget
• Tab key moves focus to next widget
• Keyboard events go to focused widget
• Mouse enter/leave tracked for hover states
• Parent widgets can intercept events

Affected Crates

lambda-rs

Notes

• Widget IDs needed for focus tracking
• Consider focus order (tab index)