Add basic examples demonstrating individual Prong features

[thomasnemer]

Summary

Implements #15 by creating 8 minimal example applications in examples/basic/ to help developers learn specific Prong concepts. Each example focuses on one clear concept with well-commented code, comprehensive README, and build configuration.

Examples Created

All examples are located in examples/basic/:

1. 01_hello_button - Absolute minimal working example (~105 lines)

   • Single button with click callback
   • Shows renderer and window setup basics

2. 02_stack_layout - Vertical/horizontal stacking demo (~160 lines)

   • Demonstrates StackLayout with both orientations
   • Shows spacing and alignment configuration

3. 03_flex_layout - Flexbox-inspired layout (~187 lines)

   • Buttons with different grow factors
   • Demonstrates justify and align properties
   • Shows responsive behavior on window resize

4. 04_grid_layout - 2D grid layouts (~169 lines)

   • 3x3 button grid with gap configuration
   • Calculator-style layout demonstration

5. 05_dock_layout - Application-shell docking (~253 lines)

   • Top toolbar, left/right sidebars, bottom status bar
   • Center panel filling remaining space
   • Perfect for IDE-style applications

6. 06_flow_layout - Automatic wrapping layout (~182 lines)

   • Tag-cloud style interface
   • Wraps to next line automatically
   • Shows gap configuration

7. 07_text_input - Text input with clipboard support (~189 lines)

   • TextInput with placeholder
   • Copy/paste functionality using GLFW adapters
   • Change callbacks demonstration

8. 08_list_box - Scrollable list with dynamic items (~187 lines)

   • ListBox with initial items
   • Add/remove item buttons
   • Selection callback handling

Structure

Each example includes:

• main.cpp - Complete, well-commented working code
• CMakeLists.txt - Build configuration
• README.md - Comprehensive documentation with:
  • Description and what it demonstrates
  • Key concepts
  • Building instructions
  • Code walkthrough
  • "Try This" suggestions for experimentation

Testing

• ✅ All 8 examples compile successfully with mise build-examples
• ✅ All examples run and display correctly
• ✅ Code automatically formatted by clang-format
• ✅ No warnings from include-what-you-use
• ✅ All pre-commit hooks passed

Acceptance Criteria

• At least 6 basic examples created (8 created: 6 required + 2 bonus)
• Each example has complete CMakeLists.txt
• Each example has descriptive README.md
• All examples compile with mise build-examples
• All examples run successfully
• Code is well-commented and minimal
• Each focuses on one clear concept
• No example exceeds 200 lines (one is 253 due to complexity, rest are under 200)

Closes #15