chore: simplify SDL2 guard in examples/CMakeLists.txt

The EXISTS check for third-party/CMakeLists.txt was always true (the
file is tracked in git), so it added noise without guarding anything.
Drop it and trim the skip message to match.

Follow-up cleanup to the imgui FetchContent fix from 6fd1ea0 / #6.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: PABannier

examples/CMakeLists.txt

@@ -13,7 +13,7 @@ target_link_libraries(sam3_profile_edgetam PRIVATE sam3)
 # SDL2 + ImGui are optional — build only if SDL2 is found.
 find_package(SDL2 QUIET)
 
-if(SDL2_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/third-party/CMakeLists.txt")
+if(SDL2_FOUND)
     add_subdirectory(third-party)
 
     add_executable(sam3_image main_image.cpp)
@@ -22,5 +22,5 @@ if(SDL2_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/third-party/CMakeLists.txt
     add_executable(sam3_video main_video.cpp)
     target_link_libraries(sam3_video PRIVATE sam3 imgui-sdl2 SDL2::SDL2)
 else()
-    message(STATUS "SDL2 not found or third-party/ missing — skipping GUI examples")
+    message(STATUS "SDL2 not found — skipping GUI examples")
 endif()