Make include-what-you-use mandatory for all example builds

[thomasnemer]

Problem

Currently, include-what-you-use (iwyu) is only enforced for the main Prong library (CMakeLists.txt:110), but not for any of the example applications. This creates an inconsistency where:

• The library code must pass iwyu checks (good!)
• Example code can have incorrect/excessive includes without triggering errors (bad!)

According to CLAUDE.md:265, we should "NEVER bypass iwyu or clang-format". While clang-format is properly enforced for all examples via the format-example-sources target, iwyu is missing.

Current State

What has iwyu:

• prong library target (via CXX_INCLUDE_WHAT_YOU_USE property)

What lacks iwyu:

• prong_demo_app executable
• All 8 basic examples (examples/basic/*/)
• All 4 intermediate examples (examples/intermediate/*/)
• All 4 advanced examples (examples/advanced/*/)
• glfw_adapters library

Why This Matters

1. Example code quality: Examples are learning resources and should demonstrate best practices
2. Include hygiene: Examples may accumulate unnecessary includes that slow compilation
3. Consistency: All code in the repository should follow the same standards
4. Documentation: Examples with incorrect includes can mislead users

Proposed Solution

Add the CXX_INCLUDE_WHAT_YOU_USE property to:

1. All example executables in individual CMakeLists.txt files
2. The glfw_adapters library
3. The prong_demo_app executable

Example configuration:

set_target_properties(01_hello_button PROPERTIES
    RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/examples/basic/01_hello_button"
    CXX_INCLUDE_WHAT_YOU_USE "${IWYU_PATH};-Xiwyu;--error;-Xiwyu;--verbose=3"
)

Affected Files

• examples/CMakeLists.txt (demo app + glfw_adapters)
• examples/basic/*/CMakeLists.txt (8 files)
• examples/intermediate/*/CMakeLists.txt (4 files)
• examples/advanced/*/CMakeLists.txt (4 files)

Total: 18 CMakeLists.txt files need updating

[thomasnemer]

Sub-tasks Created

This issue has been broken down into the following sub-tasks:

1. ✅ [#114.1] Add iwyu enforcement to glfw_adapters and demo_app #117 - Add iwyu to glfw_adapters and demo_app (size: xs)
2. ⏳ [#114.2] Add iwyu enforcement to basic examples #118 - Add iwyu to basic examples (size: s)
3. ⏳ [#114.3] Add iwyu enforcement to intermediate examples #119 - Add iwyu to intermediate examples (size: s)
4. ⏳ [#114.4] Add iwyu enforcement to advanced examples #120 - Add iwyu to advanced examples (size: s)

Recommended order: Work on these in sequence (#117 → #118 → #119 → #120)

Note: Each sub-task includes fixing any iwyu violations discovered during the build.