chore: rename project from c2pa-c to c2pa-cpp

[gpeacock]

Summary

• Updates the CMake project name from c2pa-c to c2pa-cpp (the derived ${c2pa-c_SOURCE_DIR} variables in src/ and tests/ CMakeLists fix automatically)
• Updates all GitHub repo URLs from contentauth/c2pa-c to contentauth/c2pa-cpp
• Updates all GitHub Pages doc URLs from contentauth.github.io/c2pa-c/ to contentauth.github.io/c2pa-cpp/
• Updates Doxyfile project name and brief
• Updates comment strings in scripts/amalgamate.cmake
• Updates ci-cd/amalgam-build.md title and subdirectory reference

What is intentionally NOT changed

• libc2pa_c — the native shared library name (exposes the low-level C API, stays as-is)
• c2pa-c-ffi — the Rust FFI package name (lives in a separate repo)
• c2pa_cpp — the C++ CMake target name (already correct; this rename brings the repo name into alignment with it)
• GitHub Actions workflows (contain no repo-name references)

Pre-merge checklist

• Merge this PR to main
• Rename the repo on GitHub: Settings → General → Repository name → c2pa-cpp
• Redeploy GitHub Pages docs (the Pages URL will change; the redirect does not cover GitHub Pages)
• Update the CAI documentation website (opensource.contentauthenticity.org/docs/c2pa-c slug)
• Update local git remotes: git remote set-url origin https://github.com/contentauth/c2pa-cpp.git

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