feature, add CMake find_package and add_custom_command integration

[steven-varga]

Context

The compiler currently requires manual CLI invocation. This is the #1 usability blocker. Users live in CMake; a pre-build codegen step that is not a first-class citizen in the build graph is pure friction.

Goals

1. find_package support: find_package(h5cpp-compiler) or find_package(h5cpp COMPONENTS compiler) should work.
2. add_custom_command integration: A CMake function that runs the compiler automatically when a struct header changes.
3. Check / dry-run mode: h5cpp-compiler --check to verify generated descriptors are current without overwriting. This belongs in CI.
4. Deterministic output: Generated headers should be diff-friendly (already done: #pragma once instead of random include guards).

Proposed API

find_package(h5cpp-compiler REQUIRED)

h5cpp_compiler_generate(
    TARGET my_target
    INPUTS particle.hpp simulation.hpp
    OUTPUTS particle_h5.hpp simulation_h5.hpp
    STUB_DIR ${CMAKE_CURRENT_SOURCE_DIR}/stub
)

The function should:

• Register add_custom_command so output is regenerated when inputs change
• Fail the build if compilation fails (struct not POD, missing includes, etc.)
• Optionally support --check mode for CI gating

Acceptance criteria

• cmake/FindH5CPPCompiler.cmake or config-file package
• h5cpp_compiler_generate() function documented in README
• Example project in examples/cmake-integration/
• CI job that tests the CMake integration path
• Does not break existing manual CLI usage