docs: archive completed C ABI campaign docs under plans/completed/

Move the finished migration campaign docs (handoffs v3-v6, roadmap,
R5 guides, R6 cleanup, R7 pure-ABI) from docs/zh/ into
docs/zh/plans/completed/ with an archive README; fix all
cross-references; refresh plans/README.md index (active plans now
marked unlocked).

tests/CMakeLists.txt

@@ -84,17 +84,6 @@ function(olive_add_test GROUP NAME SOURCE)
   endif()
 endfunction()
 
-include(FetchContent)
-find_package(GTest QUIET)
-if (NOT GTest_FOUND)
-  FetchContent_Declare(
-    googletest
-    URL https://github.com/google/googletest/archive/refs/tags/v1.15.2.zip
-  )
-  set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-  FetchContent_MakeAvailable(googletest)
-endif()
-
 add_subdirectory(compositing)
 add_subdirectory(general)
 add_subdirectory(timeline)