Moderinize hexer codebase

[hobu]

I saw #34 and decided the proper fix was to update the CMake configuration to use CONFIG find_package usage, so I asked Codex to do all this stuff instead.

Updates from Codex

1. Modernized the top-level CMake configuration in CMakeLists.txt.

   • Raised cmake_minimum_required to 3.16...4.3 to remove current deprecation warnings.
   • Enabled CMAKE_FIND_PACKAGE_PREFER_CONFIG.
   • Added a real WITH_TESTS option and gated enable_testing() and add_subdirectory(test) behind it.
   • Replaced legacy GDAL variable-based usage with find_package(GDAL CONFIG REQUIRED) and GDAL::GDAL.
   • Added an explicit GDAL version check for 1.9.0 or newer.

2. Updated the test build in test/CMakeLists.txt.

   • Switched GoogleTest integration from the vendored copy to FetchContent.
   • Configured GoogleTest to download from GitHub at configure time.
   • Linked tests against GTest::gtest and GDAL::GDAL.
   • Updated test registration to use $<TARGET_FILE:...>.

3. Removed the vendored GoogleTest source tree under test/gtest/.

4. Updated the local build helper script in hobu.sh.

   • Added stricter shell settings with set -euo pipefail.
   • Kept the build configured for the hexer conda environment.
   • Configured the project with Ninja and WITH_TESTS=ON.
   • Added the build step so the script now configures and compiles the project.

5. Documented the example build flow in README.md.

   • Added a Building section.
   • Documented the example command conda run -n hexer ./hobu.sh.
   • Noted that the first configure downloads GoogleTest from GitHub.

6. Added GitHub Actions CI in .github/workflows/ci.yml.

   • Added a matrix job for Linux, macOS ARM, and Windows.
   • Set up Miniforge/conda on all runners.
   • Installed cmake, ninja, and gdal.
   • Configured, built, and ran tests with CMake and CTest.
   • Added MSVC environment setup for Windows builds.