release-0.17.3

• Added SHOWCASE.md, a verified EnTT-to-cbox-to-EnTT-Pacman walkthrough
  covering cbox consumption, SDL2 provider installation, and a legacy API
  migration needed to run the game.
• CMake adoption now selects the concrete target matching project(...) when
  projects also define auxiliary header-only, C API, example, or test targets.
  It retains only that target's sources and effective unconditional metadata,
  including option-enabled source files.
• Adopted CMake libraries can derive a semantic version from an exact packed
  <PROJECT>_VERSION public-header macro, and known standard/platform headers
  no longer become misleading dependency suggestions.