Revise README and improve untracked dependency handling

[sean-parent]

Streamlines and clarifies the README with concise instructions, improved troubleshooting, and updated dependency tracking guidance. Enhances cpp-library's CMake install logic to distinguish between development and install modes for untracked dependencies: now, configuration emits warnings and uses fallbacks for local builds, but installation fails with a clear error if dependencies are not properly tracked. Adds install-time validation to prevent broken package configs and updates user guidance for mapping and tracking dependencies.

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Note

Streamlines README and updates CMake to warn and fallback on untracked deps during configure, but fail installs with new validation that prevents broken package configs.

• Build/CMake:
  • Untracked dependencies: Now lenient during configure (status notices + fallback) and strict during install (fails when _CPP_LIBRARY_IN_INSTALL_MODE is set).
  • Install-time validation: Add deferred _cpp_library_setup_install_validation that reads a generated <Package>_unverified_deps.cmake and validates before installing configs/exports.
  • Config generation: Deferred _cpp_library_deferred_generate_config now writes unverified deps file and tracks presence via globals.
  • Messaging/Guidance: Clearer errors and instructions; non-namespaced target guidance updated (map after find_package()/CPMAddPackage()).
• Docs (README):
  • Condensed/clarified setup and requirements; refined CPM/repo naming/versioning guidance.
  • Expanded dependency tracking docs with “Untracked Dependencies” section and solutions.
  • Updated examples (e.g., OpenCV mapping, BUILD_INTERFACE usage) and streamlined presets/testing/templates sections.

^{Written by Cursor Bugbot for commit e94509a. This will update automatically on new commits. Configure here.}