build: add Homebrew fallback for googletest in sandboxed environments

Adds automatic detection of Homebrew-installed googletest to support
offline/sandboxed builds (e.g., Claude Code, restricted networks).

Changes:
- Add Homebrew detection logic to cmake/dependencies/testing.cmake
- Check /opt/homebrew/opt/googletest and /usr/local/opt/googletest
- Use brew --prefix googletest for dynamic detection
- Create target aliases (gtest -> GTest::gtest) for compatibility
- Only fetch from GitHub if no local installation found
- Update macOS build docs with yaml-cpp and googletest installation

Pattern mirrors existing yaml.cmake implementation. Complements the
pre-existing yaml-cpp Homebrew fallback to minimize network dependencies.

Tested with: brew install googletest (1.17.0, 2.4MB)

Co-Authored-By: Claude <noreply@anthropic.com>

cmake/dependencies/testing.cmake

@@ -10,17 +10,86 @@ include(cmake/dependencies.lock)
 
 message(STATUS "Setting up testing dependencies with CPM.cmake")
 
-# Google Test (includes GMock)
-CPMAddPackage(
-  NAME googletest
-  VERSION ${GTEST_VERSION}
-  GITHUB_REPOSITORY google/googletest
-  GIT_TAG v${GTEST_VERSION}
-  OPTIONS
-    "BUILD_GMOCK ON"
-    "INSTALL_GTEST OFF"
-    "gtest_force_shared_crt ON"
-)
+set(_YAZE_USE_SYSTEM_GTEST ${YAZE_USE_SYSTEM_DEPS})
+
+# Detect Homebrew installation automatically (helps offline builds)
+if(APPLE AND NOT _YAZE_USE_SYSTEM_GTEST)
+  set(_YAZE_GTEST_PREFIX_CANDIDATES
+    /opt/homebrew/opt/googletest
+    /usr/local/opt/googletest)
+
+  foreach(_prefix IN LISTS _YAZE_GTEST_PREFIX_CANDIDATES)
+    if(EXISTS "${_prefix}")
+      list(APPEND CMAKE_PREFIX_PATH "${_prefix}")
+      message(STATUS "Added Homebrew googletest prefix: ${_prefix}")
+      set(_YAZE_USE_SYSTEM_GTEST ON)
+      break()
+    endif()
+  endforeach()
+
+  if(NOT _YAZE_USE_SYSTEM_GTEST)
+    find_program(HOMEBREW_EXECUTABLE brew)
+    if(HOMEBREW_EXECUTABLE)
+      execute_process(
+        COMMAND "${HOMEBREW_EXECUTABLE}" --prefix googletest
+        OUTPUT_VARIABLE HOMEBREW_GTEST_PREFIX
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+        RESULT_VARIABLE HOMEBREW_GTEST_RESULT
+        ERROR_QUIET)
+      if(HOMEBREW_GTEST_RESULT EQUAL 0 AND EXISTS "${HOMEBREW_GTEST_PREFIX}")
+        list(APPEND CMAKE_PREFIX_PATH "${HOMEBREW_GTEST_PREFIX}")
+        message(STATUS "Added Homebrew googletest prefix: ${HOMEBREW_GTEST_PREFIX}")
+        set(_YAZE_USE_SYSTEM_GTEST ON)
+      endif()
+    endif()
+  endif()
+endif()
+
+# Try to use system packages first
+if(_YAZE_USE_SYSTEM_GTEST)
+  find_package(GTest QUIET)
+  if(GTest_FOUND)
+    message(STATUS "Using system googletest")
+    # GTest found, targets should already be available
+    # Verify targets exist
+    if(NOT TARGET GTest::gtest)
+      message(WARNING "GTest::gtest target not found despite GTest_FOUND=TRUE; falling back to CPM download")
+      set(_YAZE_USE_SYSTEM_GTEST OFF)
+    else()
+      # Create aliases to match CPM target names
+      if(NOT TARGET gtest)
+        add_library(gtest ALIAS GTest::gtest)
+      endif()
+      if(NOT TARGET gtest_main)
+        add_library(gtest_main ALIAS GTest::gtest_main)
+      endif()
+      if(TARGET GTest::gmock AND NOT TARGET gmock)
+        add_library(gmock ALIAS GTest::gmock)
+      endif()
+      if(TARGET GTest::gmock_main AND NOT TARGET gmock_main)
+        add_library(gmock_main ALIAS GTest::gmock_main)
+      endif()
+      # Skip CPM fetch
+      set(_YAZE_GTEST_SYSTEM_USED ON)
+    endif()
+  elseif(YAZE_USE_SYSTEM_DEPS)
+    message(WARNING "System googletest not found despite YAZE_USE_SYSTEM_DEPS=ON; falling back to CPM download")
+  endif()
+endif()
+
+# Use CPM to fetch googletest if not using system version
+if(NOT _YAZE_GTEST_SYSTEM_USED)
+  CPMAddPackage(
+    NAME googletest
+    VERSION ${GTEST_VERSION}
+    GITHUB_REPOSITORY google/googletest
+    GIT_TAG v${GTEST_VERSION}
+    OPTIONS
+      "BUILD_GMOCK ON"
+      "INSTALL_GTEST OFF"
+      "gtest_force_shared_crt ON"
+  )
+endif()
 
 # Verify GTest and GMock targets are available
 if(NOT TARGET gtest)