Refresh to latest beman.exemplar template

[ednolan]

Summary

• Adopt exemplar's infra/ vendored infrastructure package with beman_install_library and build telemetry
• Replace vcpkg/FetchContent dependency management with lockfile.json + use-fetch-content.cmake
• Migrate CI from inline matrix to exemplar's JSON-configuration-based reusable workflows (infra-workflows@1.5.3)
• Remove C++17 from CI matrix (minimum C++ standard is C++20)
• Remove AppleClang from CI matrix (not tested in original)
• Add exemplar-standard CONTRIBUTING.md, CMakePresets.json, and pre-commit configuration
• Update README with exemplar boilerplate while preserving bounds_test-specific documentation (overview, usage examples, implementation details)
• Preserve all bounds_test-specific CMake logic: check_plat.cmake, platform-specific include directories, custom config-file package

Test plan

• Verify CI passes on all matrix entries (GCC 11-15, Clang 17-22, MSVC)
• Verify preset workflows work (cmake --workflow --preset gcc-debug, etc.)
• Verify cmake --install produces correct directory structure with platform headers
• Verify find_package(beman.bounds_test) works for consumers with platform detection
• Verify Catch2 tests still discover and pass

🤖 Generated with Claude Code

[ednolan]

Have to abandon this for now because I don't want to clobber the modules implementation until after exemplar provides its own guidance about how to use modules.