Standardize setup + run-test documentation across all Markdown docs

[Copilot]

The docs were incomplete/inconsistent for full environment setup and run test execution. This update aligns every Markdown document on a single, reproducible setup/test story with clear module vs project scope.

• Repository-wide documentation coverage

  • Expanded all existing .md files to include complete setup prerequisites and test execution guidance where applicable.
  • Updated: README.md, app/README.md, app/src/main/cpp/README.md, Projectvorstellungs.md, docs/RELEASE_CHECKLIST.md, CHANGELOG.md.

• Clear separation of scope (project vs module)

  • Root README.md remains the canonical source for full project-wide validation flow.
  • app/README.md now documents app-module test flow explicitly and links to root docs for full-project validation.
  • app/src/main/cpp/README.md is focused on native host tests and references root docs for broader Gradle validation.

• Portable command paths

  • Replaced environment-specific absolute paths with repository-relative paths for native test build output.
  • Standardized on build/native-tests for CMake/CTest examples.

• Release and overview alignment

  • docs/RELEASE_CHECKLIST.md now includes explicit setup + run test checklist items consistent with README guidance.
  • Projectvorstellungs.md references the central root test procedure to prevent drift.
  • CHANGELOG.md records the documentation expansion.

cmake -S app/src/main/cpp -B build/native-tests -DANDROIDSA_ENABLE_NATIVE_TESTS=ON
cmake --build build/native-tests --target client_state_test
ctest --test-dir build/native-tests --output-on-failure