Add GEO_UTILS_BUILD_TESTS and GEO_UTILS_BUILD_EXAMPLES options

[gistrec]

Replace the implicit CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME + BUILD_TESTING
gate with two namespaced options:

• GEO_UTILS_BUILD_TESTS (default: ON if top-level, OFF otherwise)
• GEO_UTILS_BUILD_EXAMPLES (default: ON if top-level, OFF otherwise)

Downstream consumers via add_subdirectory / FetchContent no longer build the
test executable or the examples by default. Package managers (vcpkg, conan,
distros) get a clear, project-scoped switch instead of having to override the
generic BUILD_TESTING.

Update examples/CMakeLists.txt, ci.yml, coverage.yml, CHANGELOG.md, and
docs/getting-started.md to use the new option names.

[codecov-commenter]

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