[#114.1] Add iwyu enforcement to glfw_adapters and demo_app

[thomasnemer]

Summary

Implements issue #117, part of #114 (Make include-what-you-use mandatory for all example builds).

This PR adds CXX_INCLUDE_WHAT_YOU_USE enforcement to:

• glfw_adapters library
• prong_demo_app executable

Changes

CMakeLists.txt Updates

• Added iwyu property to glfw_adapters library target
• Added iwyu property to prong_demo_app executable target
• Both targets now use --error flag to fail builds on include violations

Include Fixes

• demo_app/main.cpp: Added missing #include <string>, removed unused #include <cmath>
• viewport.h: Removed 4 unused private scrollbar state fields that triggered -Wunused-private-field warnings

Verification

All checks pass:

• ✅ mise build - No iwyu errors
• ✅ mise build-examples - Demo app builds successfully
• ✅ mise test - All 17 tests pass
• ✅ Pre-commit hooks (format-check) - Pass
• ✅ Pre-push hooks (build-validation) - Pass

Impact

• Enforces include hygiene for demo infrastructure
• Prevents future include violations in examples
• Sets foundation for adding iwyu to all other examples ([#114.2] Add iwyu enforcement to basic examples #118, [#114.3] Add iwyu enforcement to intermediate examples #119, [#114.4] Add iwyu enforcement to advanced examples #120)

Related Issues

• Closes [#114.1] Add iwyu enforcement to glfw_adapters and demo_app #117
• Part of Make include-what-you-use mandatory for all example builds #114